Annan's plan includes calls for a daily two-hour humanitarian ceasefire and access to all areas affected by the fighting in Syria.
"The Syrian government has written to the Joint Special Envoy Kofi Annan accepting his six-point plan, endorsed by the United Nations Security Council," Annan's spokesman Ahmad Fawzi said in a statement issued in Geneva.
"Mr Annan views this as an important initial step that could bring an end to the violence and the bloodshed, provide aid to the suffering, and create an environment conducive to a political dialogue that would fulfil the legitimate aspirations of the Syrian people," he said.
US Secretary of State Hillary Clinton said Syria's reported acceptance of the Annan plan must be matched by "immediate actions" like halting gunfire.
When asked what hopes she had whether President Bashar al-Assad will make good on his commitment to the plan, Clinton referred reporters to his history of "over-promising."
"Given Assad's history of overpromising and underdelivering, that commitment (to Annan) must now be matched by immediate actions," Clinton told reporters.
"We will judge Assad's sincerity and seriousness by what he does, not by what he says."
She also vowed to push the Syrian opposition for a "unified vision" that protects the rights of all Syrians when she attends weekend talks in Istanbul.
"We will be pushing them very hard to present such a vision in Istanbul," Clinton told reporters ahead of a Friends of Syria meeting on Sunday.
With diplomatic efforts to halt the bloodletting intensifying, Syria's opposition factions met for a second day in Istanbul to agree on common objectives ahead of the weekend conference.
The Syrian National Council (SNC) main opposition group unveiled a proposal to lay the foundations of a new Syria to some 400 opponents of Assad's regime, but unity proved elusive.
The National Coordination Committee for Democratic Change, which groups Arab nationalist parties, Kurds and socialists, shunned the gathering.
Annan, a former UN secretary general, held talks in Beijing with Chinese Premier Wen Jiabao, who pledged his support for his mediation efforts -- as did Russian President Dmitry Medvedev when Annan visited Moscow over the weekend.
China and Russia -- both allies of Syria -- have provoked Western fury by twice blocking UN Security Council resolutions that condemned Assad's regime.
According to Fawzi, Annan has written to Assad asking Damascus to "put its (plan's) commitments into immediate effect."
He has also urged the release of people detained in Syria over the past year.
"Mr Annan has stressed that implementation will be key, not only for the Syrian people, who are caught in the middle of this tragedy, but also for the region and the international community as a whole," Fawzi said.
Other Western nations gave a cautious welcome to the news, with most envoys saying Syria's actions now will be a test of its attitude to international calls to halt the killings.
Britain's Foreign Secretary William Hague said in London Assad's acceptance of the plan could be "a significant first step" towards ending the deadly crackdown.
"But only if it is genuinely and seriously meant," Hague said. "This has not been the case with previous commitments the regime has made."
Robert Serry, a UN Middle East peace envoy, told a Security Council meeting in New York: "Violence on the ground has continued unabated.
"Credible estimates put the probable death toll since the beginning of the uprising one year ago to more than 9,000. It is urgent to stop the fighting and prevent a further violent escalation of the conflict."
The United Nations on Tuesday gave an updated toll of "more than 9,000 people" killed, but did not specify if the deaths included soldiers and rebel troops.
The Syrian Observatory for Human Rights, meanwhile, gave a new toll of almost 10,000 people killed in violence linked to the crackdown since March last year.
A total of 9,734 people have died, including 7,056 civilians, the Britain-based monitoring group said.
Syrian forces pressed their assault nationwide on Tuesday, with at least 31 people killed, the Observatory said.
Eighteen were civilians and 13 members of the regular army, it said.
State television reported Assad had inspected troops in the flashpoint Baba Amr neighbourhood of Homs city, site of a fierce battle between regime forces and rebel troops that killed hundreds.
Thirteen of Tuesday's 13 civilian fatalities died in Homs province, the Observatory said.
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