US secretary of state Hillary Clinton has called for international backing of an arms embargo and other stronger measures enforced by the UN against Syria if the country fails to abide by a ceasefire designed to end the bloodshed on its soil.
Mrs Clinton stopped short of calling for outside military intervention in Syria - something there is little to no foreign appetite for - but said it was time to impose more consequential measures on Syrian president Bashar Assad's regime.
"We have to keep Assad off balance by leaving options on the table," she told a Paris meeting of top Western and Arab diplomats from the so-called Friends of Syria group.
Mrs Clinton's address made clear the United States has little faith in the success of special envoy Kofi Annan's peace plan. But although US policy has amounted to an acknowledgement that Assad is unlikely to be dislodged, the UN resolution Mrs Clinton seeks could strengthen Syrian rebels fighting Assad.
Any attempt to push for UN sanctions on Syria could meet resistance from Russia and China, which hold vetoes on the UN Security Council. Moscow and Beijing have already twice shielded Syria from UN sanctions over its year-long deadly crackdown on a popular uprising.
Mrs Clinton said she laid out the case for UN Security Council resolve to Russian foreign minister Sergey Lavrov, whom she said "has recognised that we are not in a static situation but a deteriorating one".
At the Paris meeting of 16 top diplomats, French foreign minister Alain Juppe said the Friends of Syria group believes Mr Annan's plan is the "last chance" for Syria to avoid civil war. Over the past 13 months, the Syrian government's crackdown is said to have killed more than 9,000 people.
Earlier, UN secretary general Ban Ki-moon said Syria was not honouring the Annan ceasefire, which took effect last week, and that violence was escalating. Syrian activists said regime forces took control of a southern town and shot at activists in another soon after international observers left.
Mr Ban recommended that the Security Council quickly approve a 300-strong UN observer mission to Syria, a number larger than was originally envisioned. But he said it will review ground developments before deciding when to deploy the mission.
He said the mission's success depends on Syria's full co-operation, and he demanded that Assad's government ensure that the observers have unrestricted freedom of movement, unfettered access to the Syrian people, and the use of helicopters and fixed-wing aircraft for their mobility. Mr Ban said the UN and Syria have signed "a preliminary protocol" in Damascus but are still discussing the use of aircraft and the nationalities of the observers. He said Syria's UN ambassador Bashar Ja'afari had assured him "there will be full support, including air mobility".