The Syrian army pressed an offensive against rebel-held areas of the central province of Homs on Monday, seeking to eliminate the last pockets of resistance to free up troops for the north.

The assault targeted two neighbourhoods of Homs city where rebel forces have been under siege for more than four months, and the nearby town of Qusayr which has been surrounded by the army since late last year, sources on both sides said.

"The army is in the midst of trying to cleanse the last rebel districts of the city of Homs," a Syrian army commander told AFP.

"The army has already cleansed the villages surrounding Qusayr, and is now trying to take back the town itself," the commander said on condition of anonymity.

A security official told AFP the army hopes to retake the besieged areas by the end of the week to free up troops for battle zones in the north, such as the commercial capital Aleppo.

"It is a huge operation, and we hope to finish it off by the end of this week," the source said, speaking on condition of anonymity.

"After that, we will concentrate on the north of Syria."

Homs province has suffered some of the worst bloodshed and destruction of the uprising which erupted against President Bashar al-Assad's regime in March last year as the army has mounted repeated attempts to recapture rebel-held areas.

The province's porous border with northern Lebanon makes it strategically important -- the rebels have used ties to sympathisers over the frontier to smuggle supplies in and wounded fighters out, while the army has sought to block the supply lines.

Homs is Syria's third largest city and the army's assault focused on the rebel-held Khalidyeh and Old City neighbourhoods, activists said.

"The army is using all kinds of weapons, and we are seeing enormous levels of destruction," an Old City-based activist who identified himself as Abu Bilal told AFP by Internet.

"Right now, there is still resistance from the (rebel) fighters, but if the army manages to enter the district, there will be a real massacre."

The government deployed fighter jets over Homs for the first time on Friday, bombing Khaldiyeh, the Syrian Observatory for Human Rights said.

In Qusayr, a town just five kilometres (three miles) southwest of Homs, activists said troops were attacking from three sides.

"The army is trying to storm Qusayr from three entrances to the town," Qusayr-based activist Hadi al-Abdallah told AFP via the Internet.

"The situation here is bad. The shelling is very, very violent," he said.

State television said the army had "restored security" to the village of Al-Atifiyeh, just outside Qusayr. "It has also killed many terrorists," it added, using the government's term for rebel fighters.

Observatory director Rami Abdel Rahman told AFP that thousands of civilians were trapped in the areas under assault by the army.