The UN-Arab envoy Kofi Annan has told the UN Security Council in Washington that Syria has not fully complied with the terms of his peace plan.
Tonight the US has drafted a resolution calling for an initial deployment of 30 unarmed observers to enter the country to report on how the fragile ceasefire is going.
Activists have posted videos showing soldiers and tanks on streets around the country, and there were reports of a fatal explosion in Homs.
Neil Connery reports from neighbouring Beirut.
Russia and China, which have twice vetoed council resolutions condemning Assad's assault on anti-government protesters, urged Damascus and the opposition to meet all the terms and conditions of Annan's plan.
Prime Minister David Cameron called for Russia and China to join with the rest of the world to "tighten the noose" on the Syrian regime.
Speaking from his South East Asia trade mission to BBC Radio 5 he said:
– Prime Minister David Cameron
This is a plan, remember, that is not just backed by those of us who have been pushing for action on Syria, it's also backed by China and Russia.
And yet Assad is deliberately flouting it. Now is the time to say to the Russians and Chinese, look at the man we are dealing with, look at the appalling way he is behaving. We need to go back to the UN and tighten the pressure, tighten the noose.
Annan said that the ceasefire appeared to be holding; but that there were unconfirmed reports of sporadic violence. He urged the Security Council to pass a motion that would enable 200 - 250 UN observers to get inside the country as soon as possible.
– UN-Arab League special envoy Kofi Annan
Syria is experiencing a rare moment of calm on the ground [that] must be sustained.
Annan's six point plan stipulates the Syrian government must:
- Commit to a Syrian-led political process to address the aspirations and legitimate concerns of the Syrian people
- Stop armed violence and immediately stop the use of heavy weapons in towns and cities; pull troops and tanks out of towns and cities
- Allow humanitarian aid to reach all areas affected by the fighting
- Release arbitrarily detained people; particularly those imprisoned for protesting
- Ensure freedom movement throughout the country for journalists
- Respect freedom of association and the right to demonstrate peacefully
Syrian UN envoy Bashar Ja'afari complained that "eight violations took place this morning by the armed (rebel) groups" and said Syria is committed to cooperating with Annan and fulfilling his peace plan.
The president of the UN Security Council for April, US Ambassador Susan Rice, welcomed the ceasefire.
Today we saw, for the first time in a long time in #Syria, a brief, positive step, which we hope will be sustained.
However she shared Foreign Secretary William Hague's concerns:
But the onus remains on the government of #Syria. Its track record up until today has been dismal.
William Hague said he was sceptical of the intentions of the Syrian government and the regime needed to shoe "visible, verifiable and indisputable signs of change"
– Foreign Secretary William Hague
I urge the Assad regime to implement fully its undertakings to the Joint Arab League and UN special envoy on Syria, including withdrawing troops and weaponry from residential areas.
The Syrian government has a record of failing to keep its promises; it has the opportunity to change that now: it should seize it. We need to see visible, verifiable and indisputable signs of change.
The opposition must also ensure that they adhere to the ceasefire and work to strengthen and broaden it."
The UN estimates around 9,000 people have died since the anti-government protests began in March 2011.