BEIRUT — Missiles fired by Syrian warplanes hit Lebanese territory Monday in one of the most serious cross-border violations since Syria's crisis began 18 months ago, security officials in Beirut and Lebanese state media said.

The officials, speaking on condition of anonymity in line with regulations, said four missiles fired by two Syrian jets hit a rugged and remote area on the edge of the Lebanese border town of Arsal. No casualties were immediately reported.

Lebanese President Michel Suleiman ordered an investigation into the border shelling Monday, without openly blaming Syria.

Lebanon's state-run National News Agency reported that the warplanes fired three missiles that fell on the outskirts of Arsal about 500 meters (yards) from the border between the two countries.

"I heard several explosions and saw four clouds of dust billowing from the area," Arsal resident Nayeh Izzedine said by telephone referring to the border. "I don't know if it was an air raid but there was a plane in the sky."

He added that the town had been quiet two hours after the 10 a.m. attack.

The Syrian forces were believed to be chasing rebels in the area, which has been the site of clashes in the past between opposition fighters battling Syrian troops just on the other side of the frontier. Lebanese armed forces have in the past detained people in the region for trying to smuggle weapons into Syria from Lebanon.

Arsal is a predominantly Sunni Muslim town, like the majority of Syria's opposition that is trying to oust President Bashar Assad from power. Assad belongs to the minority Alawite sect, an offshoot of Shiite Islam.

Syrian shells have hit Lebanese territory in the past but the air raid appears to be the most serious violation. Several Lebanese, including a journalist, have been killed and dozens wounded by fire coming from the Syrian side.

Also Monday, Syrian troops shelled rebel-held areas around the country including the northern city of Aleppo, Syria's largest, and the Damascus neighborhood of Hajar Aswad, activists said. The Britain-based Syrian Observatory for Human Rights and another activist group the Local Coordination Committees also reported clashes between troops and rebels.

On the diplomatic front, Egypt's President Mohammed Morsi held talks with the Turkish foreign minister on Syria's crisis, while Iran's top diplomat joined counterparts from the two countries later in the evening to seek an end to the war.

After the Cairo meeting, foreign ministers from the three countries said they had found common ground between them, but that a solution would not come easy. Cairo is trying to persuade Iran to drop its unquestioned support of Assad in exchange for help in easing Tehran's regional isolation, officials close to the Egyptian presidency said last week.

In Geneva, an independent U.N. panel confirmed that an increasing number of "foreign elements," including Islamic extremists, are now operating in Syria, in its first report to say that outsiders have joined a war spiraling out of control.

The investigative panel appointed by the Human Rights Council says some of these forces are joining armed anti-government groups while others are operating on their own.

"Such elements tend to push anti-government fighters toward more radical positions," the head of the panel, Brazilian diplomat and professor Paulo Sergio Pinheiro, told diplomats.

The Syrian uprising, which began with largely peaceful protests, has since morphed into a deadly armed insurgency. Hundreds of people are killed every week as the government increasingly relies on air power to try to crush the rebels.

Activists say more than 23,000 have been killed in the conflict.

The government denies that there is any popular will behind the revolt, saying it is driven by foreigners and terrorists. The regime could use the U.N. panel's report to bolster its claims.

Rebels deny that foreigners had any role starting the revolt, which was one of a series of uprisings known as the Arab Spring. Rebels say Syrians were seeking increased freedom from the autocratic regime. But as the conflict dragged on, some rebels acknowledged the presence of small numbers of foreigners among their ranks.

The U.N. panel also accused government forces and pro-regime militiamen known as "shabiha" of war crimes and crimes against humanity, including murder, summary executions, torture, arbitrary arrests, sexual violence and abuse of children. It also accused anti-government armed groups of war crimes including murder, extrajudicial execution and torture.

In a report Monday, Human Rights Watch said it documented more than a dozen extrajudicial and summary executions by opposition forces.

It said three opposition leaders who were confronted with evidence of extrajudicial executions said those who killed deserved to be killed, and that only the "worst criminals were being executed."

The New York-based group said torture and extrajudicial or summary executions of detainees in the context of an armed conflict are war crimes, and may constitute crimes against humanity if they are widespread and systematic.

___

Associated Press correspondent Aya Batrawy contributed from Cairo.

Also on HuffPost:

Captions by the Associated Pres
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>