Syria said it wants "written guarantees" that insurgents will stop fighting before it implements a troop pullback agreed by President Bashar al-Assad.

The statement puts the likelihood of an imminent truce in doubt.

Foreign Ministry spokesman Jihad Makdissi said in a statement earlier reports that Damascus "confirmed that it will pull back its troops from and around cities on 10 April is a wrong explanation."

UN-Arab peace envoy Kofi Annan, he said, "has not delivered until now written guarantees regarding the approval of terrorist armed groups to end violence and readiness to lay down its weapons."

The statement came as activists said Presidents Assad's troops were engaged in new offensives against the opposition.

Syrian forces shelled an area in the province of Idlib near the border with Turkey, killing and injuring dozens of people, opposition activists said.

Around 90 tanks and armoured vehicles, backed by helicopters, bombarded the al-Rouge Plain southwest of Idlib city, the provincial capital.

Fighters from the rebel Free Syrian Army were surrounded in al-Bashiriya, one of about 40 villages in the plain, activists added.

Reports from Syria cannot be independently verified due to restrictions on foreign media.

A major Syrian army offensive to seize back large swathes of Idlib that had fallen under rebel control has killed and wounded hundreds of people in the last 10 days.

Thousands of Syrians have fled to Turkey.

A peace plan drawn up by Mr Annan requires President Assad to "begin pullback of military concentrations in and around population centres" by Tuesday for a truce to start 48 hours later.

Mr Annan said an "unacceptable" escalation in violence in Syria violated guarantees made to him and called on the government to keep its promises to end the bloodshed.

He urged both Syrian forces and opposition fighters to halt all forms of violence by 6am Damascus time on 12 April, in line with his peace plan.

"As we get closer to the Tuesday 10 April deadline, I remind the Syrian Government of the need for full implementation of its commitments and stress that the present escalation of violence is unacceptable," Mr Annan said.

"I am in constant contact with the Syrian Government and ask all States with influence on the parties to use it now to ensure an end to the bloodshed and the beginning of dialogue," he said.