Syria announced that it will cease military operations against rebel fighters from Thursday, the day set by peace envoy Kofi Annan as the deadline for a halt to 13 months of bloodshed.
Annan said he had received a written pledge from Damascus that regime forces would halt their operations from dawn, provided they did not come under attack.
But the opposition said there was no sign of compliance on the ground by the regime as regime forces pounded protest centres anew, killing 14 civilians on Wednesday, according to monitors.
Washington said Damascus needed to match its words with deeds after repeated broken promises in the past.
"It has been decided to stop these (military) operations from Thursday morning," state television quoted an unnamed defence ministry official as saying.
The decision was taken after regime forces "completed successful operations in combating the criminal acts of the armed terrorist groups and enforced the state's rule over its territory," the official added.
But he warned that armed forces will "remain on standby to retaliate against any attack by the armed terrorist groups against civilians, the security services, armed forces, or private or public property."
Annan received a letter from Foreign Minister Walid Muallem setting out the undertaking in writing, his spokesman said.
Damascus promised to "cease all military fighting throughout Syrian territory as of 6:00 am (0300 GMT) tomorrow, while reserving the right to respond proportionately to any attacks carried out by armed terrorist groups against civilians, government forces or public and private property'."
The opposition Syrian National Council said both it and the rebel Free Syrian Army were committed to the truce but voiced scepticism about the regime's commitment.
"Similar promises were made on numerous occasions to various parties including the Arab League and major regional and international powers," the SNC said.
"The Syrian people together with the international community will be watching carefully and monitoring the regime's deeds on the ground," its foreign relations chief Bassma Kodmani added.
Washington said the Damascus regime would be judged by its actions, not its words.
"We will judge the Assad regime by what it does, not by what it says," State Department spokesman Victoria Nuland said. "Meanwhile, the violence rages on today."
The US ambassador to the United Nations, Susan Rice, went further saying Syria's promises had "little credibility" because of past broken promises.
"Fighting is still raging as we speak, reflecting what has been an intensification of the violence the Syrian government has pursued since April 1 when it committed to cease all hostile action," Rice said.
"Its commitments therefore have little, if any, credibility ... given that track record. We will await with interest the Syrian regime's posture tomorrow," the envoy added.
Syria already failed to keep a Tuesday deadline to withdraw its troops and heavy guns from population centres.
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On Wednesday, regime forces killed at least 14 civilians in bombardments of rebel neighbourhoods in the flashpoint central city of Homs and other protest hubs the Syrian Observatory for Human Rights said.
The town of Rastan, north of Homs, came under heavy shelling during the afternoon as troops clashed with rebels on its outskirts, the Britain-based watchdog said.
Speaking in Tehran, Annan said he expected the situation to be "much improved" by Thursday's ceasefire deadline if both sides respected his six-point peace plan.
"If everyone respects, I think by six o'clock (0300 GMT) on Thursday the 12th ... we a should see a much improved situation on the ground," he said.
Damascus had given "further clarifications" over how it would implement its side of the plan, he added.
Annan said his team had "had positive answers from them" and that "governments with influence" had also been approached to ensure all sides respect the ceasefire.
Iran, where Annan was speaking, is Syria's principal Middle East ally, and President Mahmoud Ahmadinejad said "all violence in the country should be halted."
"The implementation of any plan and strategy in Syria should be done without pressure and interference of others," he added.
Damascus's sympathisers on the UN Security Council, Beijing and Moscow, also piled pressure on the regime to abide by the peace plan.
"China once again calls on the Syrian government to respond to the six-point proposal," foreign ministry spokesman Liu Weimin said.
"At the same time, the Syrian opposition should also immediately stop fire and take practical measures to implement" the plan, Liu said.
Russian Foreign Minister Sergei Lavrov criticised the Syrian authorities for not heeding the peace plan more swiftly, saying "their action could have been more active (and) more decisive."
German Foreign Minister Guido Westerwelle pressed Russia to distance itself further from the Assad regime's bloody crackdown, which the United Nations says has killed more than 9,000 people since March last year.
"Russia has to make clear that it does not want to have anything to do with these acts of violence and this repression," Westerwelle said.
Annan is to brief the UN Security Council by video link from Geneva on Thursday, in what diplomats said would be "key" to the world body's next steps.
Western governments argue that a ceasefire alone is not enough and that the Damascus regime must withdraw its troops from towns and cities.
"Just ending the attacks will not be enough to be in compliance," one Western diplomat told AFP.
"It would be totally inadequate if they don't withdraw from population centres but keep troops and heavy artillery in place."