Britain, France and the United States have drafted a U.N. Security Council resolution on the deployment of U.N. observers to Syria to be discussed Thursday, French Foreign Minister Alain Juppe said.

"We fully support the plan to deploy an observer mission on the ground," said Juppe, adding that the three countries had collaborated on the text.

U.S. Secretary of State Hillary Clinton said Thursday that the ceasefire in Syria is an "important step" if it holds and urged Damascus to implement special envoy Kofi Annan's peace plan.

"If it holds, a ceasefire is an important step, but it represents just one element of the special envoy's plan," Clinton told reporters after talks with Group of Eight foreign ministers in Washington.

Presidents Nicolas Sarkozy of France and Barack Obama of the United States demanded Thursday that Syria respect the terms of a peace plan drawn up by a U.N. envoy and the Arab League.

The French presidency announced the joint call in a statement after the leaders held videophone talks on a series of world crises, and warned that Bashar al-Assad's regime would be "judged on its actions".

The U.N. Security Council could pass a resolution on Friday to allow unarmed observers to go to Syria next week to monitor the fragile ceasefire, diplomats said.

An advanced mission of 20-30 observers could be in place early next week, diplomats said. The full mission would be at least 200 monitors.

U.N.-Arab League envoy Kofi Annan said Thursday that a ceasefire in Syria appears to be holding but President Bashar al-Assad must carry out all parts of an agreed peace plan, as U.N. chief Ban Ki-moon said plans were being made to send an observer team to Syria as soon as possible.

"Syria is apparently experiencing a rare moment of calm on the ground," Annan said in a statement released as he briefed the U.N. Security Council on the 13-month-old crisis in which the U.N. says more than 9,000 people have been killed.

Syria's government on Thursday offered an amnesty to opposition gunmen without "blood on their hands," urging them to surrender as a U.N.-backed ceasefire entered into force.

"The interior ministry calls on gunmen whose hands have not been stained with Syrian blood to hand themselves in, along with their weapons, at the nearest police center. They would be released and all legal procedures against them would be terminated," it said, quoted by state television.

Russia called for more time on Thursday for international envoy Kofi Annan's Syrian peace plan to work, as a ceasefire to end months of violence came into force with no major reports of violations.

Foreign Minister Sergei Lavrov said some Arab and Western states had written off Annan's peace plan as a failure even before it had gone into effect and called on them to use their influence with the rebels to avoid future unrest.

The head of the opposition Syrian National Council on Thursday urged people to demonstrate against President Bashar Assad's regime, in a test of a ceasefire that entered in to force at dawn.

Burhan Ghalioun, who leads the exile umbrella opposition organization, also urged the international community to provide protection to Syrians who take to the streets by sending observers.

NATO said Thursday it takes its responsibility to defend allies "extremely seriously" after member state Turkey said it may seek help after shootings along its border with Syria.

"We are deeply concerned by events in Syria, particularly the recent incidents on the border with our ally Turkey," NATO spokeswoman Carmen Romero told Agence France Presse.

British Prime Minister David Cameron urged Russia and China on Thursday to join the international community and help "tighten the noose" on the Syrian regime over its deadly crackdown on protests.

A U.N.-backed ceasefire to end 13 months of bloodshed in Syria appeared to be holding after coming into force at daybreak, but doubts remained about the compliance of President Bashar Assad's government with the peace plan.