Damascus: Syrian forces killed at least seven civilians on Tuesday as they pounded protest hubs and deployed reinforcements in apparent breach of a UN-backed peace plan, activists and monitors said.

The violence drew sharp condemnation from France and Britain, with even Russia urging its ally to act more decisively to implement the truce.

Foreign Minister Walid Muallem said in Moscow that Damascus had started to carry out the plan tabled by UN-Arab League envoy Kofi Annan by pulling some troops out of certain provinces.

But France dismissed the claim as a "flagrant lie" and Britain, echoing the United States, said Syria was intensifying attacks on the opposition on the same day as the Annan plan was scheduled to begin taking effect.

A spokesman for the former UN chief, who was visiting Syrian refugee camps in Turkey, said Annan would send a letter to the Security Council later on Tuesday.

Forces loyal to President Bashar al-Assad shelled the villages of Marea and Hawr al-Nahr in northern Aleppo province, said the Syrian Observatory for Human Rights.

Mortar rounds also struck old quarters of the flashpoint city of Homs, killing six people in Khaldiyeh neighbourhood while a seventh person was shot dead elsewhere in the central city.

A total of 17 people died on Tuesday across Syria, including six soldiers killed by gunmen in the northeastern province of Hassakeh, the Britain-based monitoring group said.

Regime forces also shot four people in the village of Kfar Zeita, in the central Hama province, but it was not clear if they were rebels or civilians.

Clashes between regime forces and rebels also rocked the district of Mzeyreeb, in the southern province of Daraa, the cradle of the dissent movement launched a year ago.

The Local Coordination Committees, one of the main opposition groups inside Syria, said "large military reinforcements" had arrived overnight on the eastern outskirts of Rastan, in the central province of Homs.

The report could not be verified due to curbs on media activity.

Under the peace plan it agreed with Annan, Syria is supposed to draw back its troops and armour from population centres on Tuesday ahead of a complete ceasefire on Thursday.