Eight members of Syria's military intelligence were killed by an Islamist militant car bomb on Thursday night near the southern frontier with the Israeli Golan Heights, opposition activists and a violence monitoring group said on Friday.

The Syrian Observatory for Human Rights said the bomb was planted by Al-Nusra Front, a rebel unit fighting to oust President Bashar Assad that the United States has labelled a terrorist group.

"We think the blast might have killed a colonel who has been leading the fight against rebels in the area," Rami Abdelrahman, head of the Britain-based Observatory said. The building targeted is in the town of Saasa, 14 miles (23 km) from the frontier with the Golan Heights, he said.

He added that death toll was likely to rise as several security personnel were in a critical condition.

Syrian rebels have been battling Assad's army for months in towns inside and adjacent to the Area of Separation between Israel and Syria, along the disengagement line from the 1973 war.

State-run Israel Radio aired what it said was an interview with an unnamed man from Saasa saying the explosion was close to Israel. "I heard an explosion. I did not see it, I heard it. It was a very large explosion," he said.

Meanwhile on Friday, 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 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.