BEIRUT -- Activist groups said Sunday that about 5,000 people were killed in Syria's civil war in August, the highest figure ever reported in more than 17 months of fighting as President Bashar Assad's regime unleashed crushing air power against the revolt for the first time.

The U.N. children's fund UNICEF put the death toll for last week alone at 1,600, the largest weekly figure for the entire uprising.

"The past month witnessed large massacres and the regime was conducting wide operations to try to crush the uprising," said Omar Idilbi, a Cairo-based activist with the Local Coordination Committees group. "Last month's acts of violence were unprecedented."

He said the increased use of the air force and artillery bombardments was behind the spike in casualties.

The civil war witnessed a major turning point in August when Assad's forces began widely using air power for the first time to try to put down the revolt. The fighting also reached Syria's largest city, Aleppo, which had been relatively quiet for most of the uprising.

The Britain-based activist group Syrian Observatory for Human Rights said that 5,440 people, including 4,114 civilians were killed in August. The LCC put the toll at 4,933 civilians.

On Sunday, the Observatory and the LCC said more than 100 people were killed throughout Syria and the groups have been reporting 100-250 deaths per day over the past week.

Syria's uprising has been the bloodiest in the Arab Spring that has already removed long-serving authoritarian leaders in Egypt, Tunisia, Yemen and Libya.

The two main activists groups also released new death tolls for the entire uprising since March 2011. The Observatory said more than 26,000 have been killed, including more than 18,500 civilians. The LCC put the death toll at more than 23,000 civilians. The LCC does not count members of the military who are killed, but the Observatory does.

That averages out to about 1,300-1,500 deaths per month, making the August figure more than three times higher than average.

The groups had previously reported a toll of around 20,000 more than a month ago.

On Thursday, Human Rights Watch said government forces have killed scores of civilians over the past three weeks by bombarding at least 10 areas where they were lining up to buy bread at bakeries near and around Aleppo.

Last week, activists reported that between 300 and 600 people were killed in the Damascus suburb of Daraya during days of shelling and a killing spree by troops who stormed the town after heavy fighting.

"The reason behind the high death toll is military operations, shelling, clashes and air raids," said Rami Abdul-Rahman who heads the Observatory.

"I would say most people are being killed during clashes and executions," he said referring to scores of bodies that are found in streets around Syria who are shot execution style with a bullet in the back of their heads.

UNICEF spokesman Patrick McCormick said 1,600 were killed last week alone, including some children. He did not immediately explain how he arrived at the figure, but said the number was documented.

As the death toll mounted, international efforts to end the crisis faltered badly. The U.N. and Arab League have both led prolonged but ultimately failed efforts to negotiate an end to the violence.

Turkey this week called for the U.N. to authorize creation of a safe zone in Syria for tens of thousands fleeing their homes. Britain and France have left open the possibility of more aggressive action, including a military-enforced no-fly zone to protect a safe area – though that still seems a remote possibility.

Lakhdar Brahimi, the U.N.'s new envoy to Syria, told Assad's regime on Saturday that change is both "urgent" and "necessary" and that it must meet the "legitimate" demands of the Syrian people, words that will not win the seasoned Algerian diplomat and international trouble shooter any friends in Damascus.

On his first day on the job, Brahimi also called on both sides to end violence in Syria, but said Assad's government bears more responsibility than anyone else to halt the bloodshed.

While the military largely has been able to quell the offensive rebels launched in Damascus in July, it is still struggling to stamp out a rebel push in the northern city of Aleppo.

In the latest violence on Sunday, the Observatory said the military pounded rebel holdouts in Aleppo, the country's commercial capital. There was also fighting in other areas including the central city of Homs, Idlib province on the border of Turkey and suburbs near Damascus.

The Observatory said 21 people were killed when troops stormed the village of Alfan in the central province of Hama. It added that eight people were killed in the oil-rich eastern province of Deir el-Zour that borders Iraq.

An amateur video posted online showed more than a dozen bodies in Alfan covered with white shrouds in accordance with Islamic tradition as men and women sat around them crying and hugging the dead. A woman opened the shroud to see the face of one of the dead, then kissed it and started weeping.

Loud screams of prayers could be heard in a hall that appeared to be inside a mosque.

In the capital Damascus, two bombs exploded near the Syrian military's joint chiefs of staff offices, lightly wounding four army officers and damaging buildings and cars, state television reported. The twin blasts in the posh Abu Rummaneh district were the latest in a wave of bombings to hit Damascus in recent months as clashes between government troops and rebels reached the tightly controlled capital.

There was no immediate claim of responsibility for the bombings, which government officials said appeared to target a building under construction near the offices of the joint chiefs of staff. The building, which is officially known as the Guards Battalion and was empty at the time of the blast, serves as a base for army officers who guard the joint chiefs of staff offices some 200 meters (yards) away.

Several past bombings have targeted the security establishment in Damascus, most notably a July blast that killed four senior security officials, including the defense minister and his deputy, who was Assad's brother-in-law.

The government officials, speaking on condition of anonymity because they are not authorized to brief the media, said the wounded Sunday were army officers and they had minor injuries.

Footage broadcast on state TV showed a damaged building with debris strewn across the street. The blasts punched a hole in one of the building's walls, and blew out the windshield and windows of an SUV parked nearby.

The twin bombing was the second in recent weeks to hit Abu Rummaneh. On Aug. 15, a bomb attached to a fuel truck exploded outside the Dama Rose hotel where U.N. observers stayed before ending their mission to Syria. That blast, which hit a military compound parking lot, wounded three people.

Late Saturday, a car bomb near a Palestinian refugee camp in a suburb of Damascus killed at least 15 people, according to state news agency SANA. It said Sunday the explosion in the suburb of al-Sbeineh also wounded several people and caused heavy damage to buildings in the area.

It blamed the blast on an "armed terrorist group," the term the regime uses to describe the rebel Free Syrian Army seeking to topple Assad.

When Syria's unrest began, the country's half-million Palestinians at first struggled to remain on the sidelines. But in the past months, young Palestinian refugees – enraged by mounting violence and moved by Arab Spring calls for greater freedoms – have been taking to the streets and even joining the rebels.

___

Associated Press writer Albert Aji in Damascus contributed to this report.

Also on HuffPost:

Loading Slideshow...
  • Syrian Regime

    Despite major defections and a July 18. explosion in Damascus that killed four top generals, including President Bashar Assad's brother-in-law, the regime's inner circle is still powerful and united against the opposition. Assad's inner circle includes his younger brother, Maher, who commands the forces in charge of protecting the capital. It also includes the heads of the four intelligence agencies playing a major role in the crackdown. Although regime forces lost parts of the northern city of Aleppo, Syria's largest, government troops still control most cities, while the opposition dominates large parts of the countryside. <em>Caption: This June 13, 2000, file photo shows Syrian President Bashar Assad, right, his brother Maher, center, and brother-in-law Major General Assef Shawkat, left. (AP Photo, File)</em>

  • Free Syrian Army

    The main rebel fighting force for more than a year, the Free Syrian Army includes lightly-armed volunteer militiamen and defectors from Assad's military. Its overall strength and structure is unclear, but tens of thousands are believed be loyal to the group. The rebels have control over some northern areas, allowing movement of fighters and supplies from Turkey and Lebanon. Anti-Assad forces have failed to maintain any strategic footholds in big cities, being driven back from key neighborhoods in Homs earlier this year and now apparently losing ground in the largest urban center, Aleppo. The battles also suggest only weak direction from central commanders - including Turkey-based Free Syrian Army leader Riad al-Asaad. <em>Caption: In this citizen journalism image provided by Shaam News Network SNN, taken on Sunday, Aug. 12, 2012, Free Syrian Army soldiers pose for a photograph, in Sarmada, Idlib province, northern Syria. (AP Photo/Shaam News Network, SNN)</em>

  • Syrian National Council

    Based in Istanbul, the SNC has emerged as the main political opposition to Assad and has pushed for international recognition as the legitimate representative of the uprising, despite rifts with other Syrian factions. The group also has been hit by internal feuds that have led some senior members to quit. The current leader, Abdelbaset Sieda, is a Swedish-based activist for Syria's minority Kurdish community. The SNC has gained support from many countries in the West and Arab world, but it has not galvanized international backing, and critics complain its senior leadership is made up mostly of exiles out of touch with their homeland. <em>Caption: The members of the Syrian National Council and its head Abdulbaset Sieda, center, arrive for a meeting with Turkey's Foreign Minister Ahmet Davutoglu in Ankara, Turkey, Monday, July 23, 2012.(AP Photo/Burhan Ozbilici)</em>

  • The National Coordination Committee for Democratic Change

    A rival to the SNC, the National Coordination Committee is led by opposition figures inside Syria, many of them former political prisoners. SNC members accuse the group of being far too lenient and willing to engage in dialogue with the regime. In turn, the National Coordination Committee accuses the SNC of being a front for Western powers and willing to open the door to the Muslim Brotherhood and other conservative Islamist factions. <em>Caption: Member of the National Coordination Committee for Democratic Change, Morhaf Mickael speaks during a meeting of Syrian opposition parties in Brussels on Sunday, June 24, 2012. (AP Photo/Virginia Mayo)</em>

  • International Alliances

    On Assad's side are traditional Shiite allies Iran and Lebanon's Hezbollah. <em>Caption: In this photo released by the Syrian official news agency SANA, Iran's Supreme National Security Council, Saeed Jalili, meets with Syrian President Bashar Assad in Damascus, Syria, Tuesday, Aug. 7, 2012. (AP Photo/SANA)</em>

  • International Alliances

    The regime also has important political cover from Russia and China, which have used their Security Council vetoes to prevent U.N. sanctions on Syria. <em>Caption: In this Jan. 25, 2005 file photo, Syrian President Bashar Assad, left, and Russian President Vladimir Putin shake hands during a signing ceremony in the Kremlin, Moscow. (AP Photo/Sergei Chirikov)</em>

  • International Alliances

    The rebels have built an array of regional support that includes the wealthy Gulf states - led by Iran rival Saudi Arabia - and neighboring Turkey, which offers key supply routes. The West also backs the rebel forces, but has so far opposed mobilizing international military support similar to the NATO-led airstrikes that helped topple Moammar Gadhafi's regime in Libya. <em>Caption: From left, Bahrain's Foreign Minister, Sheik Khalid bin AhmedI bin Mohammed al-Khalifa, Saudi Arabia's Foreign Minister Prince Saud al-Faisal, his Turkish counterpart Ahmet Davutoglu and United Arab Emirates' Sheikh Abdullah bin Zayed al-Nahayan seenduring a group photo during the Gulf Cooperation Council (GCC) Foreign ministers meeting in Istanbul, Turkey, Saturday, Jan. 28, 2012 (AP Photo)</em>

  • Foreign Fighters

    Syria has drawn foreign fighters just as other recent conflicts such as Iraq and Afghanistan. No credible count on them exists, but anecdotal evidence suggests foreigners are coming to fight Assad. Rebel commanders downplay the presence of foreign fighters, saying their cause is a purely Syrian uprising. Mohammed Idilbi, a Syrian activist based in Turkey, says foreign ranks include Libyans, Yemenis, Tunisians and Lebanese. On Saturday, Syria's official SANA news agency claimed four Libyans were among rebels killed in Aleppo. <em>Caption: In this Sept. 18, 2011 file photo, former rebel fighters celebrate as smoke rises from Bani Walid, Libya, at the northern gate of the town. (AP Photo/Alexandre Meneghini, File)</em>

  • Extremists

    U.S. officials and others worry that Syria could become a new foothold for insurgents inspired by al-Qaida. Assessing the degree of radical Islamic ideology in the civil war is impossible, but at least one group, the al-Nusra Front, has emerged and declared allegiance to the Free Syrian Army. Al-Nusra, or Victory, has claimed responsibility for several high profile attacks, including a double suicide bombing in March that killed 27 people in Damascus and the execution-style killing of a Syrian television presenter who was abducted in July. On Friday, U.S. intelligence officials said al-Qaida has advanced beyond isolated pockets in Syria and now is building a network of well-organized cells that could include several hundred militants. <em>Caption: This photo shows Al-Qaida's new leader Ayman al-Zawahiri in a still image from a web posting by al-Qaida's media arm, as-Sahab, Wednesday July 27, 2011. Al-Qaida's new leader has lauded protesters in Syria for seeking to topple the regime of President Bashar Assad. (AP Photo/IntelCenter) </em>