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 the key border town of Qusair 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 United Nations and opposition groups to warn of a humanitarian catastrophe.

The rebels, fighting to topple President Bashar al-Assad, have suffered a string of setbacks recently, despite shipments of more advanced weapons from Gulf Arab countries. The powerful Hezbollah was instrumental in the government’s victory in Qusair, and opposition activists say its fighters are helping army troops in Homs and even parts of Aleppo, farther north.

Emboldened, the government has tried to build on its successes. Last weekend, it launched a major offensive on Homs, a central city of about 1 million on the road between the capital, Damascus, and regime strongholds on the coast.

The fighting in Syria has increasingly taken on sectarian undertones, with Assad backed by 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, 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 pro-Assad forces were attacking the area from three sides with rocket launchers, tanks and mortars.

— Associated Press