The United Nations-Arab League envoy to Syria, Kofi Annan, has said there is still time for a ceasefire in the country.

Mr Annan was speaking at a news conference in Turkey, after visiting some of the camps housing an estimated 25,000 people who fled from Syria to escape the violence.

The UN Security Council this evening called on Syrian President Bashar al-Assad to keep this Thursday's deadline for a complete ceasefire in Syria.

All 15 council members have expressed "deep concern" at the Syrian government's failure to implement today's deadline to pull its troops out of cities and towns so a ceasefire can take place.

Meanwhile, Syrian forces pressed home a sustained assault on opponents of President Assad, ignoring an international peace plan under which troops were to silence their guns and withdraw from urban areas.

Opposition groups said Syrian troops killed 31 people on Tuesday, and Turkish media reported heavy gunfire coming from what appeared to be an army post topped with a Syrian flag a short distance from the Turkish border.

Reports from Syria cannot be independently verified as the government has barred international journalists and rights groups.

"Every effort must be made to achieve a cessation of violence in all its forms on 12 April at 0600 (4am Irish time)," Mr Annan told the UNSC in a letter.

"There is no more time to lose," Mr Annan said in a statement to the press. "We must all push for an end to the bloodshed before Syria plunges into the abyss."

US Ambassador to the United Nations Susan Rice, president of the council this month, said all council members voiced "deep concern" at Damascus' level of commitment to its truce pledges.

Mr Annan said he had information the Syrian military was withdrawing from some areas but moving to others not previously targeted.

Shelling of restive parts of Homs killed at least 26 people today and five died in violence elsewhere, opposition groups said, but there was no sign of a military pullout, with tanks still in cities such as Homs and Hama.

Opposition activists say more than 800 Syrians have been killed since Mr Assad accepted Mr Annan's peace proposals on 27 March.

Citing satellite images, a French foreign ministry spokesman denounced a Syrian assurance that troops were withdrawing as a "blatant lie".

Foreign Minister Alain Juppe added: "Assad lied to Kofi Annan, who has the total support of the international community."