Western powers have reacted sceptically to Syria's acceptance of the peace plan put forward by UN envoy Kofi Annan.
US Secretary of State Hillary Clinton said President Bashar al-Assad would be judged by events.
"Given Assad's history of over-promising and under-delivering, that commitment must now be matched by immediate actions," she said.
Earlier the UN said more than 9,000 people have now died since the Syria uprising started a year ago.
"If he is ready to bring this dark chapter in Syria's history to a close he can prove it by immediately ordering regime forces to stop firing and begin withdrawing from populated areas," Mrs Clinton said.
Other Western powers have made similar comments.
Mrs Clinton also said President Assad must allow in humanitarian aid and start preparing for a democratic transition.
And she said opposition groups "must come forward with a unified position, a vision of the kind of Syria they are wishing to build".
Opposition factions meeting in Istanbul have called on Syria to withdraw its tanks, to show it is serious about accepting Annan's proposals, Agence France-Presse has reported.
The BBC correspondent at the gathering says he has not met a single delegate who thinks the president is even remotely sincere, and Mr Assad's opponents would never accept any deal allowing him to remain in power.
Most opposition groups have now agreed to name the Syrian National Council as the formal representative of the Syrian people.
But our correspondent says their disunity was openly on display, with constant disputes and walkouts.
Mr Annan's spokesman Ahmed Fawzi said he considered the Syrian acceptance of the peace plan as "important initial step" to help bring an end to the violence, but added that implementation was key.
Mr Annan thanked countries that have supported his attempts to mediate in the conflict. He is currently in Beijing for talks with Chinese Premier Wen Jiabao.
The six-point plan calls for Mr Assad's government to pull troops and heavy weapons out of population centres, and for all parties to allow for a daily two-hour pause to the fighting in order for humanitarian aid to reach affected areas. The plan also requests that the authorities release those detained in the uprising.
However, it does not impose any deadline for Mr Assad, or call for him to leave power.
The BBC's Barbara Plett at the UN says despite the scepticism, this is a new situation because it is the first strategy for ending the conflict that has the backing of the entire Security Council, including Syria's allies Russia and China.
She says it seems it was this international unity which forced Bashar al Assad to accept the plan.
Mr Annan has written to Mr Assad urging him to put his commitments into immediate effect.
the BBC's Middle East editor Jeremy Bowen says the agreement is not as strongly worded as earlier UN resolutions, but it can be regarded as more pressure on Mr Assad and his government, which he seems to have decided that he cannot ignore.
However, implementing the plan is another matter, our correspondent adds.