The United Nations expects a ceasfire between Syrian troops and opposition forces to be in place no later than April 12, a spokesman for the U.N.-Arab League envoy Kofi Annan said Thursday, as activists reported a fierce attack by government troops on a Damascus suburb.

Kofi Annan's peace plan had previously set the April 10 deadline for Syrian troops to pull out of towns and cities.

Annan's spokesman Ahmad Fawzi said they expect both sides to end the hostilities within 48 hours of the already agreed April 10th deadline.

"The clock starts ticking on the 10th for both sides to cease all forms of violence," he said.

Fawzi said that by then the Syrian military units should withdraw from populated centers and no new units should move into cities.

Activists, meanwhile, said Syrian troops were attacking a suburb of the Syrian capital, Damascus, in what they described as one of the most violent campaigns since the year-old uprising began.

Syria has told U.N. officials that it is withdrawing troops from the southern province of Daraa, the northwestern province of Idlib and the rebel-controlled mountain resort town of Zabadani, north of Damascus, Fawzi said.