BEIRUT -- Syrian government troops unleashed a major artillery barrage on 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 UN 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.
The regime has tried to build on its successes to further shore up its military position. It launched a major offensive last Saturday on Homs, a central city of about 1 million located 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 Shia Islam, while the rebels are mainly Sunnis.
Activists, who consider Homs "the capital of the revolution," say the regime wants to capture the city to include it in a future Alawite state stretching to the coast, where many believe Assad would take refuge in a last resort.
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.