Britain's Foreign Secretary has promised tough measures against the Syrian regime should the fighting continue beyond this morning's agreed ceasefire.
"This is a very important moment," William Hague said.
"If there isn't a ceasefire or it isn't upheld, in the view of the United Kingdom, we will want to return to the UN Security Council - and intensify our support for the Syrian opposition."
Mr Hague's comments come as widespread doubts linger that Syria would observe the UN-brokered ceasefire, after fierce attacks on opposition neighbourhoods in the run-up to the deadline.
The Syrian government has pledged to halt its military attacks ahead of the dawn deadline but says it will still respond to any rebel attacks.
The promise of compliance came in a letter to the UN and Arab League envoy Kofi Annan amid reports of new assaults by regime forces in several towns and cities.
Shelling was reported in the town of Rastan, while at least 20 government tanks were said to have moved into the centre of Hama just hours before the 4.00am - British summer time - deadline for the fighting to stop.
Mr Annan's spokesman said the letter from the Syrian foreign ministry made no mention of any troop withdrawal from urban areas, despite a pull-back being one of the key points of the agreed plan.
The regime said it would "cease all military fighting throughout Syrian territory", but also said it would respond to any attacks by "armed terrorist groups".
It raises the possibility of Syrian forces remaining in towns and cities and blaming rebel fighters - who have also pledged to abide by the truce - for any violence.
There are no independent observers to monitor the deal, although Mr Annan's six-point peace plan includes the possibility of a team of around 200 unarmed UN troops being sent in as monitors.
Syrian activists say at least 20 people were killed in attacks by government forces across the country on Wednesday.
Videos posted online showed neighbourhoods in Homs being bombarded by tank fire and artillery earlier on Wednesday.
Attacks were also reported in the southern town of Deraa and in several suburbs of the capital, Damascus.
Syria's compliance with the deadline will be judged by the UN Security Council later today.
The body issued a statement last week endorsing the plan and raising the possibility of the consideration of "further measures" if the violence continues.