Syrian forces pressed home a sustained assault on opponents of President Bashar al-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.
Peace envoy Kofi Annan appealed to the U.N. Security Council to use its leverage to prevent the collapse of his efforts to halt 13 months of conflict and said Assad must make a fundamental change of course and adhere to a ceasefire due to begin on Thursday.
Every effort must be made to achieve a cessation of violence in all its forms on 12 April at 0600 (04:00 a.m. British Time), Annan told the council in a letter, seen by Reuters.
There is no more time to lose, Annan said in a statement to the press. We must all push for an end to the bloodshed before Syria plunges into the abyss.
U.S. 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.
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 on Tuesday 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 Assad accepted Annan's peace proposals on March 27.
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.
The White House also saw no sign of a pullback: Leaders of the Assad regime ... make a lot of promises, spokesman Jay Carney said. Those promises overwhelmingly turn out to be empty.
Nor did rebels immediately stop shooting. The anti-Assad Syrian Observatory for Human Rights said insurgents killed six soldiers in attacks on checkpoints on an eastern desert road.
State media reported the funerals of 33 security personnel on Tuesday, bringing to 58 the number it said had been killed in two days. Syrian government media curbs make it hard to verify reports from inside the country.
Syrian Foreign Minister Walid al-Moualem demanded guarantees from Annan that rebels would also honour any truce.
We will not ask the terrorist groups, which are killing, kidnapping and destroying infrastructure, for guarantees. We want Annan to give us these guarantees, Moualem said during a visit to Moscow.
The last-minute demand, a variant of one Syria made at the weekend, is not mentioned in Annan's proposals and looked designed to complicate his struggle to get all parties to comply with a six-point plan that appears so far to be a dead letter.
The rebel Free Syrian Army will fight on if Assad fails to withdraw troops and tanks from in and around cities as required, a spokesman, Colonel Qassem Saad al-Deen, told Reuters.
The opposition Syrian National Council said Syrian forces were not complying and that world powers should impose an arms embargo and other measures if the peace plan failed.
More time means more blood, council spokeswoman Basma Kodmani told reporters in Geneva. It is urgent to end the regime's repression and the regime itself.
China, which along with Russia has blocked punitive U.N. Security Council action against Syria, said it hoped all sides would immediately obey the U.N.-backed ceasefire, aimed at stopping the uprising from sliding into full-scale civil war.
Russian Foreign Minister Sergei Lavrov said he had told Moualem that Syria could be more active, more decisive in meeting the terms of Annan's plan, but he also urged foreign states to lean on opposition groups to stop shooting.
Moualem said some troops had already pulled back from cities in line with the peace plan, but he tied a full ceasefire to the entry of foreign monitors, another apparently new condition.
Assad's forces have killed more than 9,000 people in the past year, according to a U.N. estimate. Damascus says rebels have killed more than 2,500 soldiers and security personnel.
The violence has alarmed Syria's neighbours, especially Turkey which already hosts almost 25,000 Syrian refugees. At least five people, including two Turkish citizens, were wounded by cross-border fire into a refugee camp in Turkey on Monday.
(Additional reporting by Khaled Yacoub Oweis and Jonathon Burch in Antakya, Ben Blanchard in Beijing, Jonathon Burch in Turkey, Stephanie Nebehay in Geneva, Adrian Croft in London, Steve Gutterman in Moscow, Dominic Evans, Douglas Hamilton, Mariam Karouny and Erika Solomon in Beirut and Louis Charbonneau at the United Nations; Editing by Giles Elgood)