In some of the heaviest fighting of a six-week-old ceasefire, at least 11 civilians are reported dead in Syria's second-largest city, Aleppo, after government forces and rebels traded rocket fire.

Six civilians were killed and eight injured when the government of Bashar Al-Assad launched airstrikes on the rebel-held part of Aleppo. Rebels responded with rocket fire into the government-controlled district in the western part of the city. That left five more dead, and 20 injured, according to the Syrian Observatory for Human Rights.

"There's a clear escalation," said Observatory head Rami Abdel Rahman. "This was the bloodiest incident in Aleppo and its province" since the ceasefire took effect on February 27. "This escalation directly threatens the truce."

The ceasefire between the government and nonjihadist rebels was brokered by Russia and the United States. But the agreement does not include Islamic militants such as the Islamic State or al Qaeda's local splinter group.

The fledgling UN-backed peace negotiations continue.

Despite accusations of violations on both sides, the ceasefire has largely held. But there are growing fears that if it does crumble, Aleppo will be the flashpoint, partly because militants are involved in the fighting there, too.

IS advances on rebels

Rebel groups have lost territory to IS jihadists in recent days, who are threatening Azaz, a key opposition town just 5 miles (8 kilometers) from the Turkish border.

The militant attacks have forced 30,000 Syrians to flee their homes in recent weeks and put tens of thousands more at risk.

Syria's civil war erupted in 2011, and has so far killed more than 270,000 people. Half of its prewar population of 23 million has been displaced, including 5 million refugees who have fled abroad.

Most have fled to neighboring states such as Turkey, Jordan and Lebanon, but around a million have sought refuge in Europe.

bik/bk (AFP, SOHR)