Peace envoy Kofi Annan said a ceasefire in Syria appeared to be holding on its first day on Thursday, as the Syrian government and its foes traded charges of trying to wreck his peace plan.

Yet Annan said Syrian President Bashar al-Assad had not fulfilled the UN-backed peace plan and called on the UN Security Council to demand that troops be pulled out of cities.

"I am encouraged by reports that the situation in Syria is relatively calm and that the cessation of hostilities appears to be holding," the UN-Arab League envoy said in a statement released as he briefed the Security Council.

But he insisted that "all parties have obligations to implement fully the six-point plan. This includes both the military provisions of the plan and the commitment to move to a political process."

"What has happened today does not constitute full compliance by the Syrian government," Annan was quoted as saying. "Syrian troops and armour must return to their barracks immediately."

The plan calls for the withdrawal of forces from urban areas, the release of arbitrarily detained people, freedom of movement for journalists and the right to demonstrate.

Despite the regime's commitment to pull back, the spokeswoman for the opposition Syrian National Council (SNC), Basma Qoudmani, said "we have concrete proof that heavy weapons are still in population centres."

And the Syrian Observatory for Human Rights said that, by mid-afternoon, it had not seen any troop withdrawals.

Spokeswoman Qoudmani and Facebook site "Syrian Revolution 2011" also called for peaceful demonstrations across Syria to test the government's readiness to accept public shows of dissent.

"The real test will be if there is shooting or not when people demonstrate," Qoudmani said.

The interior ministry said people wanting to demonstrate must have permits.

"The right to demonstrate peacefully is guaranteed by law. We call on citizens to apply the law by requesting a permit before demonstrating," said a statement carried by the official SANA news agency.

The ministry said the measure is aimed at "securing the safety of citizens and to practise this right in a civilised manner."

Presidents Nicolas Sarkozy of France and Barack Obama of the United States also demanded that Syria respect the terms of the peace plan.

"The two presidents urged the Syrian regime to scrupulously and unconditionally respect its undertakings in regard to the plan by the United Nations special envoy and the Arab League," a French statement said.

"In liaison with their partners, notably their Arab partners, they agreed to intensify their efforts, including at the UN Security Council, to bring a definitive end to the brutal repression of the Syrian people," it said.

The leaders vowed to find a way "for humanitarian aid to be delivered, and for the Syrian people to freely choose their destiny."

And they warned: "Those responsible for abuses will have to answer for their crimes."

UN chief Ban Ki-moon said plans were being drawn up to send observers to Syria, starting with the dispatch of a UN peacekeeping general as early as Friday.

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.

Ban said "the world is watching however with sceptical eyes," adding that previous promises made by the regime "have not been kept."

In Washington, French Foreign Minister Alain Juppe said a draft resolution on the deployment of observers will be submitted on Thursday.

And long-time Syrian ally Russia said the Security Council could pass it by as early as Friday.

Russian ambassador Vitaly Churkin said Moscow wanted to see observers in Syria as early as next week.

SNC chief Burhan Ghalioun urged the international community to "monitor its implementation in full, mainly the right to demonstrate ... and to provide the means to protect the people if the regime violates the plan."

"International observers should be on the ground as soon as possible," he said.

Renewed bloodshed on Thursday killed at least eight people, including seven civilians, and wounded dozens more, the Britain-based Observatory said.

Even so, the toll is markedly lower than it has been in recent weeks, when there have often been scores of people killed.

State media charged that it was the opposition who were jeopardising the truce, accusing rebels of bombing a bus ferrying troops to their base in Syria's second-largest city Aleppo.

"An armed terrorist group used an explosive device to target a bus transporting officers and non-commissioned officers to their unit in Aleppo. It killed a lieutenant colonel" and wounded 24 others, state news agency SANA said.

The interior ministry urged tens of thousands of people who fled the violence both inside and outside the country to return home and offered an amnesty to opposition gunmen without "blood on their hands."

The rebel Free Syrian Army, for its part, insisted it was "100 percent committed" to the ceasefire in a conflict monitors say has killed more than 10,000 people since March last year.

"The regime is being elusive. We are 100 percent committed to the ceasefire, but the regime is not abiding by it," FSA spokesman Colonel Kassem Saadeddine told AFP by Internet.

Saadeddine denied any involvement in the attack on the bus, dismissing the report as regime propaganda "to avoid fulfilling its commitment."

In other developments, Juppe said France has "elements of evidence" that crimes against humanity have been committed by the Syrian regime.

"France has gathered a certain number of elements of evidence which would enable us if the time comes, notably at the UN, to take it before the international courts, because crimes against humanity have been committed," he told reporters on the sidelines of a meeting of G8 ministers.