Kofi Annan has urged the UN Security Council to help prevent the collapse of his efforts to end Syria's year-long unrest, after Damascus ignored a deadline to withdraw troops and stop shelling towns.
"Every effort must be made to achieve a cessation of violence in all its forms on 12 April at 06:00 (03:00 GMT)," Annan, the UN-Arab League envoy for Syria, told the council on Tuesday in a letter.
"There is no more time to lose," Annan said in the letter. "We must all push for an end to the bloodshed before Syria plunges into the abyss."
US Secretary of State Hillary Clinton said in Washington that "intensive discussions" on Syria were under way in New York and in key capitals. "I will be particularly raising this with [Russian] Foreign Minister [Sergei] Lavrov and the council will hear directly from Kofi Annan on Thursday," she said.
"We will have another go at trying to persuade the Russians that the situation is deteriorating and the likelihood of regional conflict and civil war is increasing."
While Annan is now in the Iranian capital Tehran for talks with the Syrian ally, Clinton and Lavrov plan to meet in Washington on Wednesday.
Susan Rice, the US ambassador to the UN who heads the Security Council this month, said all of its members had voiced "deep concern" at Damascus' level of commitment to its truce pledges.
Rice said council members would "face a moment of truth" when they will have to decide whether to increase pressure on the government of Syrian President Bashar al-Assad, who has shown no sign of complying with a Tuesday deadline to withdraw forces from towns and stop using heavy weapons.
Annan was far from optimistic in his assessment of the situation in Syria in his letter to the Security Council.
"The days before 10 April should have been an opportunity for the government of Syria to send a powerful political signal of peace, with action on all aspects of the six-point [peace] plan," he wrote.
Damascus had agreed to the Security Council-backed Tuesday deadline, to be followed by a full ceasefire by the army and opposition fighters on Thursday morning.
Annan made clear there was still a chance to end the violence by Thursday's deadline. "The Syrian leadership should now seize the opportunity to make a fundamental change of course."
He said the opposition should also cease fighting in order to "give no excuse for the government to renege on its commitments."
Earlier on Tuesday, Walid al-Muallem, the Syrian foreign minister, had 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," Muallem 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 his truce plan.
In-depth coverage of escalating violence across Syria
The Russian foreign minister told Muallem 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.
Muallem 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.
Colonel Qassem Saad al-Deen, a spokesman for the opposition Free Syrian Army, told Reuters that it will fight on if the government fails to withdraw troops and tanks from in and around cities as required.
Meanwhile, 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.
The Syrian Observatory for Human Rights also said rebels 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.
Assad's forces have killed more than 9,000 people in the past year, according to a UN estimate. Damascus says rebels have killed more than 2,500 soldiers and security personnel.
Syrian government media curbs make it hard to verify reports from inside the country.