BBC
The Syrian authorities have made no mention of troop withdrawals from urban areas
The US said Syria's pledge held "little if any credibility".
The main armed rebel group said it would not stop shooting if government troops continued to 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.
But an announcement on state TV made no mention of a withdrawal from urban areas which, under the Annan plan, should have been completed by Tuesday.
Commitments 'broken' US ambassador to the UN Susan Rice described the caveats in the letter as "worrying" and added that the onus was on the Syrian regime to halt the violence.
Following a phone call between German Chancellor Angela Merkel and US President Barack Obama, the two leaders said they "shared the concern that the Assad government was not complying with the terms of the agreement negotiated by Kofi Annan".
But Russia - one of Syria's closest allies - said the rebels must respond with their own ceasefire.
US Secretary of State Hillary Clinton will meet her Russian counterpart Sergei Lavrov in Washington DC on Thursday during a G8 foreign ministers summit.
'Regime won't stop' Meanwhile, the violence continued on Wednesday, especially in the city of Homs.
Activists said at least 30 people were killed in offensives around the country, including the north and north-west, a valley west of Damascus, and in the south, near Deraa.
A spokesman for the main rebel force, the Free Syrian Army (FSA), said the ceasefire was unlikely to take effect because neither side would stop shooting.
"If the other side stopped, the Syrian people would march on the president's palace on the same day. This means the regime won't stop," Captain Ayham al-Kurdi said in a BBC interview on the Turkey-Syria border.
The BBC's UN correspondent, Barbara Plett, says Mr Annan has been in contact with Iran, as well as Russia, asking them to use their influence.
If he fails, some Council members have said they will push for tougher action - not military intervention, but perhaps sanctions, our correspondent says.
Some influential voices in the US Congress have also been calling for the Syrian opposition to be armed.
President Obama has opposed that course of action, but on Wednesday he met the defence minister of Saudi Arabia, which supports arming the opposition.
Any further action would need the agreement of Russia and China, who have already vetoed two Security Council resolutions on Syria.
Mr Annan is due to brief the UN Security Council later on Thursday.
The UN estimates about 9,000 people have died since anti-government protests began in March 2011. In February, the Syrian government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.
The Syrian authorities have made no mention of troop withdrawals from urban areas A ceasefire for Syria proposed by UN-Arab League envoy Kofi Annan is due to come into force in the next few hours, but Western countries have publicly doubted Syria's willingness to comply.
On Wednesday, Syria said it would abide by the ceasefire but reserved the right to respond to attacks. The US said Syria's pledge held "little if any credibility".
The main armed rebel group said it would not stop shooting if government troops continued to 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.
But an announcement on state TV made no mention of a withdrawal from urban areas which, under the Annan plan, should have been completed by Tuesday.
Commitments 'broken' US ambassador to the UN Susan Rice described the caveats in the letter as "worrying" and added that the onus was on the Syrian regime to halt the violence.
Continue reading the main story 
A huge amount of pressure has been mounted to bring about the agreement of all sides to the Annan truce.
Above all, Russia must have exerted powerful influence behind the scenes to induce the change of tune by the Damascus regime.
China and Iran, Syria's other two important international friends, have also strongly backed the Annan mission and may have helped bend President Bashar al-Assad's ear.
Mr Annan's priority was to stop the carnage. If that can be achieved, and stabilised by the insertion of UN observers, huge challenges will remain - above all, working towards an workable political settlement.
Again, Russia is poised to play a crucial role. Much will depend on what vision it has for Syria's future - perhaps regime mutation rather than the regime change sought by the opposition and its western backers.
Analysis
Above all, Russia must have exerted powerful influence behind the scenes to induce the change of tune by the Damascus regime.
China and Iran, Syria's other two important international friends, have also strongly backed the Annan mission and may have helped bend President Bashar al-Assad's ear.
Mr Annan's priority was to stop the carnage. If that can be achieved, and stabilised by the insertion of UN observers, huge challenges will remain - above all, working towards an workable political settlement.
Again, Russia is poised to play a crucial role. Much will depend on what vision it has for Syria's future - perhaps regime mutation rather than the regime change sought by the opposition and its western backers.
British Foreign Secretary William Hague said that if there was a ceasefire, the UK would work at the United Nations to send monitors to Syria urgently "to make sure they can't slide back to renewed conflict".
"If there isn't a ceasefire... we will want to return to Security Council in a new attempt to obtain a resolution on Syria, we'll intensify our support for the opposition and we will seek stronger sanctions," he added. Following a phone call between German Chancellor Angela Merkel and US President Barack Obama, the two leaders said they "shared the concern that the Assad government was not complying with the terms of the agreement negotiated by Kofi Annan".
But Russia - one of Syria's closest allies - said the rebels must respond with their own ceasefire.
US Secretary of State Hillary Clinton will meet her Russian counterpart Sergei Lavrov in Washington DC on Thursday during a G8 foreign ministers summit.
'Regime won't stop' Meanwhile, the violence continued on Wednesday, especially in the city of Homs.
Activists said at least 30 people were killed in offensives around the country, including the north and north-west, a valley west of Damascus, and in the south, near Deraa.
A spokesman for the main rebel force, the Free Syrian Army (FSA), said the ceasefire was unlikely to take effect because neither side would stop shooting.
Footage purporting to show clashes in Homs on Wednesday has been posted online
The BBC's UN correspondent, Barbara Plett, says Mr Annan has been in contact with Iran, as well as Russia, asking them to use their influence.
If he fails, some Council members have said they will push for tougher action - not military intervention, but perhaps sanctions, our correspondent says.
Some influential voices in the US Congress have also been calling for the Syrian opposition to be armed.
President Obama has opposed that course of action, but on Wednesday he met the defence minister of Saudi Arabia, which supports arming the opposition.
Any further action would need the agreement of Russia and China, who have already vetoed two Security Council resolutions on Syria.
Mr Annan is due to brief the UN Security Council later on Thursday.
The UN estimates about 9,000 people have died since anti-government protests began in March 2011. In February, the Syrian government put the death toll at 3,838 - 2,493 civilians and 1,345 security forces personnel.
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