Syrian troops heavily shelled the northwestern protest city of Idlib early Saturday, in an apparent prelude to a ground assault just hours before the scheduled arrival of international peace envoy Kofi Annan, a watchdog and an activist said.

"It's the heaviest bombardment since troop reinforcements were sent to Idlib earlier this week," the head of the Britain-based Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP.

"It's the prelude to the launch of a ground offensive" in Idlib, whose province of the same name borders Turkey and where the rebel Free Syrian Army is entrenched.

Local activist Milad Fadl said: "The bombardment began at 5 am (0300 GMT). The shelling is very, very heavy.

"There are already three buildings that have collapsed in Thalathin Street in the west of the city," added Fadl, a member of the opposition Syrian Revolution General Commission.