Syria has told Kofi Annan, the special peace envoy of the UN and Arab League, that it will halt all fighting by Thursday morning but reserves the right to respond to any attack by "armed terrorist groups", his spokesman said.
In a letter the Syrian Foreign Ministry said Damascus agreed "to cease all military fighting throughout Syrian territory as of 6 am (0300 GMT) tomorrow, Thursday," Ahmad Fawzi said.
The statement also said that the Syrian government reserves "the right to respond proportionately to any attacks carried out by armed terrorist groups against civilians, government forces or public and private property".
Annan said he would continue to work with the government and opposition to ensure complete implementation of his six-point peace plan.
Live Box 20117693233259422
The government's pledge to stop army operations means the opposition must now also honour the ceasefire, Russia's deputy foreign minister said.
"The Syrian government has declared it will cease fire as of 6 am on April 12. Now it's up to the armed opposition," Deputy Foreign Minister Gennady Gatilov said on Twitter. "Those are the conditions of the Annan plan."
Al Jazeera's Cath Turner, reporting from the UN in New York, said the statement by the Syrian government "has been greeted with a great deal of suspicion and cynicism".
"Susan Rice, the US ambassador to the UN, and the current president of the UN Security Council, spoke about the letter that has been submitted by president Assad, and she was very clear in the fact that she was not taking anything in that letter at its word, because, she said, President Assad did not have a very good track record at keeping his word," our correspondent said.
Meanwhile, Annan has welcomed Iranian support for his efforts to secure peace in Syria, telling Tehran that it can be "part of the solution".
Annan was speaking in Tehran on Wednesday following talks with Ali Akbar Salehi, the Iranian foreign minister.
||Commitment to stop violence by all parties
||Inclusive Syrian-led political process
||Daily two-hour humanitarian ceasefire
||Media access to all areas affected by the fighting
||Release of people detained arbitrarily
||Right to demonstrate
While endorsing Annan's peace plan, which calls for the ceasefire by Thursday, Salehi said Syria's government needed to be given time to implement reforms.
Tehran is considered a key regional ally of Syrian President Bashar al-Assad, who faces growing international pressure over the crackdown by security forces that has seen cities shelled and thousands of people killed.
Annan stressed again the urgency of finding a way to end the killing and to provide humanitarian assistance to those in need, before getting all parties to the table.
"The political process must be Syrian-led and respect the aspirations of the Syrian people," Annan said.
"What is important is that governments in the region and beyond work with Syria to resolve the crisis.
"The geopolitical position of Syria is such that any miscalculation can have unimaginable consequences."
'Further militarisation disastrous'
Regarding a ceasefire agreement which requires Syrian government forces to halt operations by April 12, Annan said he had received assurances that the deadline would be honoured.
"If everyone respects it I think by six in the morning on Thursday we shall see improved conditions on the ground," Annan said.
Answering a question whether he supported calls by some countries, such as Saudi Arabia and Qatar, to arm the Syrian opposition, Annan said "any further militarisation will be disastrous".
Salehi offered qualified Iranian support for Annan's efforts.
"We believe the people of Syria, like other countries, have the right to enjoy all the rights enjoyed by other world nations, such as freedom of political parties, freedom of elections, a constitution that encompasses all the wishes of a nation," he said.
"At the same time, we have announced that we oppose interference in the affairs of all countries, including Syria.
"The government of Bashar al-Assad has promised change to meet the demands of the people... and in fact the opportunity must be given to the Syrian government."
Annan's peace plan, presented last month, calls on the Syrian government to withdraw troops from towns and end the use of heavy weaponry.
Under the plan, both the army and opposition fighters must adhere to the ceasefire.
Walid al-Muallem, the Syrian foreign minister, on Tuesday demanded guarantees from Annan that the opposition fighters 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.
Syria failed to observe a Tuesday deadline to withdraw its forces from urban areas, and activists reported fresh violence on Wednesday.
Live Box 2011421105226899357
The Local Co-ordination Committees said there was shelling of several opposition-held neighbourhoods in the central city of Homs.
The UK-based Syrian Observatory for Human Rights said "tens of army vehicles" were deploying in the southern town of Maaraba amid intense shooting.
The Syrian National Council's spokeswoman Basma Kodmani said that if Assad does not show signs of adhering to the ceasefire, the UN Security Council must set an ultimatum with the will to enforce by power.
"What we would like to see is a unanimous decision by members of the Security Council that sends an ultimatum to the regime with a deadline that is not too far down the road that says on such and such a date enforcement measures will intervene", Kodmani told Reuters.
Assad's forces have killed more than 9,000 people in the past year, according to a UN estimate. Damascus says "armed terrorist groups" have killed more than 2,500 soldiers and security personnel.
Source: Al Jazeera and agencies