A small advance group of UN monitors is preparing to go to Syria to oversee the ceasefire, hours after the Security Council voted for its deployment.
UN Secretary General Ban Ki-moon said he would make firm proposals in days for a larger group of about 250 people.
The mission comes as a BBC reporter says the ceasefire appears in danger of collapsing in some parts of Syria.
Activists said at least 20 people were killed as violence flared on Saturday in Homs and at a funeral in Aleppo.
The US ambassador to the UN, Susan Rice, condemned the bloodshed saying it raises "renewed doubts about the sincerity of the [Syrian] regime's commitment to the ceasefire".
She said the resolution was an important opportunity to stop the bloodshed, adding that the burden was now on the Syrian regime.
But Syria's ambassador to the UN, Bashar Ja'afari, blamed opposition forces for the spike in violence, saying that more than 50 violations had taken place including "many assassinations and sabotage operations".
Syria's opposition umbrella group, the Syrian National Council, welcomed the vote.
Freedom of movement
The resolution was passed unanimously after Russia approved a revised text, which authorised the deployment of a team of about 30 unarmed observers.
Mr Ban said the UN would need complete freedom of movement for its monitors.
"I know that it is a very big country so we will try to have a very effective way of monitoring the situation there," he said.
Diplomats had revised a US-proposed draft on Friday to accommodate Russian objections.
Russia's ambassador said Moscow supported the resolution because of the need for a rapid deployment of observers.
Indeed the UN has said that it intends to increase the deployment to 250 - but that is dependent on certain conditions and will require further approval.
Meanwhile the BBC's Jim Muir in Beirut, who has been monitoring developments in Syria, says that in parts of the country the ceasefire seems to be in danger of collapsing, unless so
is done to shore it up.
Saturday witnessed a surge in violence, with activists saying at least 20 people killed in several incidents around the country.
Onus on government
Mr Annan, the envoy for the UN and Arab League, drew up the plan which called for an advance monitoring team to be deployed immediately to Syria to observe compliance with the truce.
Mr Annan's plan aims to end more than a year of violence in Syria which the UN says has killed more than 9,000 people, mostly civilians.
The resolution passed on Saturday expresses an intention to establish a full mission once there is a sustained cessation of violence. It also puts the onus on the Syrian government to meet its ceasefire commitments.
The text "calls upon all parties in Syria, including the opposition, immediately to cease all armed violence in all its forms."
The BBC's Barbara Plett at the United Nations in New York says that it provides important backing to Mr Annan's peace plan.
Significantly, it is the first time Security Council members have been able to overcome divisions and adopt a resolution on Syria, which is a diplomatic defeat for Damascus, she says.
Analysts say Russia appears to have been key to persuading President Bashar al-Assad to accept both the Annan plan and the ceasefire.
The terms of this resolution, which Russia backed, call on all parties to observe that truce - and exerts even more pressure on Syria's leaders to withdraw their tanks and forces even further.