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.

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.

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 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 oldest child was 5 years old.

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

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.