A view of the damaged Sheikh Kamel Mosque in the old city of Homs

Syrian artillery hit parts of Homs city and at least 37 people were killed in clashes around Syria on Friday, opposition activists said, as peace envoy Kofi Annan told President Bashar al-Assad his forces must be first to cease fire and withdraw.

"The deadline is now," Annan's spokesman, Ahmad Fawzi said in Geneva. "We expect him to implement this plan immediately."

An army pullback to bases would permit a safe return to mass, peaceful protest, said anti-government activists. But there was no sign of any risk-free demonstrations on Friday, reports Reuters.

Heavy clashes raged in several provinces, including the north-eastern Deir al-Zor province. Seven civilians died in fighting there, which also killed two rebels and three soldiers, the Syrian Observatory for Human Rights reported.

In the capital Damascus, Friday protests drew gunfire in several neighbourhoods. Two died in the crossfire between rebels and state forces, the British-based Observatory said.

"Clashes erupted between armed defector groups and the regime forces in the Jobar neighbourhood of Damascus after security forces tried to break up a protest," it said.

In Homs, Syria's third city, residents said shells and mortar rounds exploded as troops raided anti-Assad areas, killing 10 civilians. Farther north, in the business hub of Aleppo, an explosion badly wounded six soldiers and five civilians, activists said.

Assad vowed that he would spare no effort to implement Annan's peace proposals, but warned they would not work unless there is an end to foreign funding and arming of rebel groups.

The United Nations says Assad's forces have killed at least 9,000 people in the year-long uprising. The government says around 3,000 soldiers and police have been killed.

The leader of Lebanon's powerful Shi'ite group Hezbollah, one of Assad's main allies, said the world would no longer demand the president's ouster and that rebels were incapable of toppling him by force.

"The armed opposition is incapable of toppling the regime," Sayyed Hassan Nasrallah said. "Some people talked about the political option...but with conditions that equalled the fall of the regime, for example for President Assad to step down. I think the international and regional political climate today has passed this phase."

Nasrallah pointed to Annan's initiative as proof that world powers would not try to help the opposition topple Assad.

Removing any ambiguity about the ceasefire terms of the peace plan, Annan's spokesman said it was up to the Syrian military to move first and show good faith by withdrawing tanks, big guns and troops from cities.

The Annan plan "specifically asks the government to withdraw its troops, to cease using heavy weapons in populated centres", Fawzi said. "The very clear implication here is that the government must stop first and then discuss a cessation of hostilities with the other side and with the mediator."