BEIRUT — Syrian government troops unleashed a major artillery barrage on the city of Homs on Friday, hitting buildings near a 13th century mosque as they pressed an assault on rebel-held areas in the country's strategic heartland.

Opposition activists said Lebanese Hezbollah guerrillas had joined the fighting in Syria's third largest city. If confirmed, it would be the first major involvement for the Iranian-backed group since it helped regime troops capture a key border town from the rebels last month.

As the shells landed, thousands of civilians trapped in the city faced severe shortages of food, water and medicine, prompting the U.N. and opposition groups to warn of a humanitarian catastrophe.

The rebels fighting to topple President Bashar Assad have suffered a series of setbacks recently, including the loss of the strategic town of Qusair near the Lebanese border last month. Despite recent shipments of more advanced weapons from Gulf Arab countries, they have been unable to score any major gains in the past few weeks.

The powerful Hezbollah, a staunch ally of the regime, was instrumental in the regime's victory in Qusair. Opposition activists say the group's fighters have spread out in Homs and even parts of Aleppo in the north, propping up outstretched army troops.

Emboldened, the regime has tried to build on its successes to further shore up its military position. On Saturday, it launched a major offensive on Homs, a central city of about 1 million located on the road between the capital Damascus and regime strongholds on the Mediterranean coast.

The fighting in Syria has increasingly taken on sectarian undertones as Assad enjoys support from many in his Alawite sect, an offshoot of Shiite Islam, while the rebels are mainly Sunnis.

Activists, who consider Homs "the capital of the revolution," say the regime wants to capture the entire city to include it in a future Alawite state stretching to the coast, where many believe Assad would take refuge in a last resort.

"It appears the regime wants to take Khaldiyeh (district), no matter what the price," said Tariq Badrakhan, an activist based in the northern district of Homs where the fighting was concentrated Friday. He said troops backed by pro-government militiamen and Hezbollah fighters were attacking the area from three sides with multiple rocket launchers, tanks and mortars.

Online video showed shells slamming into buildings in the densely built-up area near the historic Khalid Ibn al-Walid mosque, famous for its nine domes and two minarets that tower over the skyline. The video, posted on Saturday, appeared consistent with AP's reporting from the area.

The Syrian Observatory for Human Rights, a U.K.-based group that tracks the civil war in Syria through activists on the ground, said a woman and her three children were killed in the shelling on Khaldiyeh Friday evening. The eldest child was aged five.

The Observatory also said Hezbollah guerrillas were fighting in Homs, although Hezbollah has not commented and the reports could not be independently verified.

The militant group only acknowledged its participation in the Qusair battles after dozens of its fighters were returned home dead. There has been no sign of that in the past few days.

Homs has been an opposition stronghold since the early days of the uprising against Assad. Mass Arab Spring-inspired protests there starting in early 2011 prompted repeated army offensives on the city. Hundreds of people were killed and thousands driven from their homes.

The government now controls much of the city, though several neighborhoods in the center including Khaldiyeh and Bab Houd are still opposition strongholds.

Badrakhan said government forces were facing stiff resistance from the rebels and have not advanced far in six days of fighting.

But Mohammed Saleh, a Homs-based Alawite activist, said regime forces would be able to capture all-rebel-held areas in Homs within days as rebels are surrounded and running low on ammunition.

He said the regime was trying to strengthen its hand ahead of any peace talks by capturing as much territory as possible.

"The capture of Homs means that an international political solution is closer," Saleh said, referring to attempts by the United States and Russia to bring rival factions in Syria to a peace conference.

Rupert Colville, spokesperson for the U.N. High Commissioner for Human Rights Navi Pillay, said he is "extremely concerned about the human rights and humanitarian impact" of the government offensive.

"Shortages of food, water, medicine, electricity and fuel in besieged areas are severely affecting civilians, including women and children," he said in a statement issued in Geneva.

Recent reports suggest that armed opposition groups are operating inside residential areas, increasing the risk for civilians, the statement said, adding that the number of civilians trapped in the heavy fighting in and around Homs is believed to be between 2,500 and 4,000.

In Turkey, Syria's main opposition bloc appealed to the United Nations and Western countries that have supported the opposition in Syria's civil war "to intervene immediately" and provide food and medicine to the besieged, rebel-held areas of Homs.

"The areas under attack in Homs have been cut off from the rest of the world and suffer from an urgent shortage of medicine and food," the Syrian National Coalition said in a statement.

More than 93,000 people have been killed in the conflict that began as peaceful protests but turned into an armed revolt after opposition supporters took up arms to fight a brutal government crackdown.

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  • March 2011: 120

    Thousands of Syrians rally to show their support for President Bashar al-Assad, who is facing unprecedented domestic pressure amid a wave of dissent, in Damascus on March 29 2011. (ANWAR AMRO/AFP/Getty Images)

  • April 2011: 820

    A protestor burns a portrait of Syrian President Bashar al-Assad during a demonstration after Friday prayers on April 29 2011 in Istanbul against the regime of al-Assad and the deadly crackdown on opposition protests. (BULENT KILIC/AFP/Getty Images)

  • May 2011: 850

    A veiled woman takes part in a protest calling on Syria's President Bashar Assad to step down, in front of the United Nations headquarters in Amman, on May 21 2011. (KHALIL MAZRAAWI/AFP/Getty Images)

  • June 2011: 1,000

    Syrian refugees arrive to a makeshift camp in the northern city of Idlib, in Syria, on June 13 2011. (MUSTAFA OZER/AFP/Getty Images) <em><strong>CORRECTION:</strong> An earlier version of this caption placed the city of Idlib in Turkey. Idlib is in Syria. </em>

  • July 2011: 1,600

    Thousands of pro-regime Syrians wave their national flag and portraits of President Bashar al-Assad during a rally in Damascus on July 17 2011. (LOUAI BESHARA/AFP/Getty Images)

  • August 2011: More than 2,000

    People hold pictures of Syrian president Bashar al-Assad and fallen Libyan strongman Muammar Gaddafi as they take part in a demonstration gathering activists opposed to Syria's regime of President Bashar al-Assad on August 28 2011 at Taksim Square in Istanbul. (BULENT KILIC/AFP/Getty Images)

  • September 2011: 2,700

    A national flag hangs on a statue of Syria's late president Hafez al-Assad at the entrance of the flashpoint city of Homs on August 30 2011, as rights activists reported widespread anti-regime protests across Syria on the first day of the feast marking the end of Ramadan. (JOSEPH EID/AFP/Getty Images)

  • October 2011: 3,000

    Supporters of Syrian President Bashar al-Assad wave Syrian flags during a pro-regime rally in Damascus on October 12, 2011. Assad's regime is facing international pressure amid a violent crackdown on anti-government protests that broke out in March across Syria. (LOUAI BESHARA/AFP/Getty Images)

  • November 2011: More than 4,000

    Syrian soldiers carry on November 26, 2011 the coffin of a comrade reportedly killed in an ambush by an armed group in the flashpoint Syrian city of Homs. (AFP/Getty Images)

  • December 2011: More than 5,000

    Free Syrian Army captain identified as Ahmed al-Arabi sits in a safe house near Wadi Khaled on the Lebanese-Syrian border on December 30 2011. (Si Mitchell/AFP/Getty Images)

  • January 2012: 7,100

    Free Syrian Army fighters take position in a house on the Lebanese-Syrian border prior to a nighttime operation on January 2 2012. (Si Mitchell/AFP/Getty Images)

  • February 2012: 7,500

    A Turkish journalist in Ankara, holds pictures of two journalists, French photojournalist Remi Ochlik and Sunday Times correspondent Marie Colvin, killed in an alleged rocket attack by Syrian regime forces against a makeshift opposition media center in the besieged city of Homs in Syria on February 22 2012. (ADEM ALTAN/AFP/Getty Images)

  • March 2012: More than 8,000

    A defected Syrian soldier, now a member of the Free Syrian Army, stands outside a mountain outpost near the village of Janudieh in the northern province of Idlib on March 20 2012. (STR/AFP/Getty Images)

  • April 2012: Close to 9,000

    Syrians pray over the bodies of Syrian violence victims at a funeral in the northwestern town of Kafr Zeta on April 10, 2012. (AFP/Getty Images)

  • May 2012: More than 9,000

    A general view shows the Syrian flag flying next to destruction in the Bab Amro neighbourhood of Homs on May 2 2012. (JOSEPH EID/AFP/GettyImages)

  • June 2012: 14,000

    A Syrian man carries a wounded girl next to Red Crescent ambulances following an explosion that targeted a military bus near Qudssaya, a neighbourhood of the Syrian capital, on June 8, 2012. (AFP/GettyImages)

  • July 2012: 19,000

    Members of Jihadist group Hamza Abdualmuttalib train near Aleppo on July 19, 2012. Rebels seized control of all of Syria's border crossings with Iraq on July 19. (BULENT KILIC/AFP/GettyImages)

  • August 2012: 23,000

    A Syrian boy whose family has been displaced due to fighting between rebel fighterws and Syrian government forces is seen near the Syrian border with Turkey on August 25, 2012. (ARIS MESSINIS/AFP/GettyImages)

  • September 2012: 30,000

    A Syrian man carrying grocery bags tries to dodge sniper fire as he runs through an alley near a checkpoint manned by the Free Syria Army in the northern city of Aleppo on September 14, 2012. (MARCO LONGARI/AFP/Getty Images)

  • October 2012: 36,000

    A Syrian boy plays on a destroyed tank near the rubble of a mosque that was destroyed during fighting between Syrian rebels and regime forces in the northern city of Azaz on September 23, 2012. (MIGUEL MEDINA/AFP/GettyImages)

  • November 2012: 40,000

    A walnut tree stripped of its branches stands in the rubble of the Kalat al-Numan citadel, originally built during the Roman era some 2000-years-ago, after allegedly being bombed several times by the Syrian air force on November 18, 2012, in Maaret Al-Numan in southern Idlib province. (John Cantlie/AFP/Getty Images)

  • December 2012: 60,000

    Smoke rises in the Hanano and Bustan al-Basha districts in the northern city of Aleppo on December 1, 2012 as fighting continues through the night. (Javier Manzano/AFP/Getty Images)

  • January 2013: 65,000

    A boy plays with a balloon in a Syrian refugees camp in Azaz, near the Turkish border, on January 10, 2013 after snow falls. (EDOUARD ELIAS/AFP/Getty Images)

  • February 2013: More than 70,000

    Syrian Zakia Abdullah sits on the rubble of her house in the Tariq al-Bab district of the northern city of Aleppo on February 23, 2013. (Pablo Tosco/AFP/Getty Images)