By Khaled Yacoub Oweis

AMMAN (Reuters) - The Syrian army has stepped up an offensive on opposition Sunni Muslim strongholds in the central city of Homs, bringing in ground forces to try to secure passage for its forces through a major road junction, opposition sources said on Friday.

Around 15,000 Sunni civilians were trapped on the southern and western edge of the city on Friday near the intersection of Syria's main north-south and east-west arteries, crucial to let Assad's forces travel between Damascus and the Mediterranean coast, opposition campaigners in Homs said.

They said army rocket, artillery and aerial bombardment had killed at last 120 civilians and 30 opposition fighters since Sunday.

Syrian authorities have banned most independent media, making it difficult to verify such reports on the ground.

The sources also said shabbiha militiamen loyal to President Bashar al-Assad had killed more than 100 Sunni men, women and children when they overran a nearby area 10 days ago.

Activist Nader al-Husseini, speaking by phone from the western sector of Homs, said at least 10,000 shabbiha had been brought from the coastal city of Tartous to back up the regular army.

"They go in infantry formations behind the soldiers and their specialty is looting and killing civilians," he said, adding that among dozens killed by the shabbiha were a family of five in the village of Naqira.

Husseini said 100 wounded civilians were trapped in Homs' western neighborhood of Kafar Aya and that the Free Syrian Army rebels had tried to negotiate a deal to evacuate them but failed.

Mostly Sunni Homs, a commercial and agricultural hub 140 km (90 miles) north of Damascus, has been at the heart of the uprising and armed insurgency against Assad and his establishment, composed mostly of Alawites, who follow an offshoot of Shi'ite Islam and comprise about 10 percent of the population.

The nearly two-year-old conflict has now killed an estimated 60,000 people.

Syrian authorities have not commented directly on the latest offensive, but official media have in the past referred to the need to ‘cleanse' the city of what they described as terrorists who were terrorizing peaceful neighborhoods.

(Reporting by Khaled Yacoub Oweis, Amman newsroom; Editing by Kevin Liffey)