A ceasefire for Syria proposed by UN-Arab League envoy Kofi Annan has come into force, but Western countries have publicly doubted Syria's willingness to stick to it, - BBC reported.
Syria has said it would abide by the ceasefire but reserved the right to respond to attacks.
So far there have been no reported violations, but the US said Syria's pledge had "little if any credibility".
The main armed rebel group said it would respond to any government attack.
Mr Annan received written assurances from the Syrian foreign ministry that government troops would "cease all military fighting throughout Syrian territory as of 06:00 (03:00 GMT)... while reserving the right to respond proportionately to any attacks carried out by armed terrorist groups", his spokesman, Ahmad Fawzi said.
"An hour after the ultimatum expired, the situation is calm in all regions," Rami Abdel Rahman, chairman of the London-based Syrian Human Rights Observatory, told the AFP news agency.
If the ceasefire does hold, the focus is expected to fall on the withdrawal of government troops, tanks and heavy weaponry. That step was supposed to have been completed on Tuesday, the BBC's correspondent in Beirut, Jim Muir, says.
