Syrian forces have renewed their week-long assault on Homs, activists have said, in the heaviest bombardment the city has seen since the country's uprising began in March.

Hundreds of people have been reported killed since last Friday in a barrage of rockets, mortars and machine-gun fire. The Syrian Observatory for Human Rights said at least 12 people were killed on Thursday morning but an exact death toll could not be determined because of the chaos in the city.

President Bashar Assad's regime is trying to crush pockets of dissent in the city. Many areas have been under the control of army defectors who want to bring down the regime by force.

Homs, Syria's third-largest city, has become the focus of both resistance and reprisal in the 11-month uprising as many areas have fallen under the control of increasingly bold army defectors who want to bring down the regime by force.

In the latest operation, which began on Saturday, government forces have unleashed a relentless offensive, shelling residential areas as they try to root out any resistance and retake control of the city.

The uprising began with mostly peaceful protests, but it has transformed into an armed insurgency against Assad in many areas. The UN estimates that 5,400 people have been killed in Syria since the uprising began in March. But that figure is from January, when the UN stopped counting because deteriorating security prevented verification of the figures.

UN Secretary-General Ban Ki-moon has said the head of the Arab League plans to send observers back into Syria and has raised the possibility of a joint mission with the United Nations. The UN chief provided no specifics, but the idea appears aimed at giving the regional group a boost after the league's earlier mission was pulled out of the country because of security concerns.

"In the coming days we will further consult with the council before fleshing out details," Mr Ban said. "We stand ready to assist in any way that will contribute toward improvement on the ground."

Mr Ban also reiterated his "deep regret" over the council's inability to speak in one voice to stop the bloodshed. Russia and China used their veto powers on Saturday to block a Security Council resolution backing an Arab League peace plan that calls for Assad to step aside.

He added: "Thousands have been killed in cold blood, shredding President Assad's claims to speak for the Syrian people. "I fear that the appalling brutality we are witnessing in Homs, with heavy weapons firing into civilian neighbourhoods, is a grim harbinger of worse to come."