Russia and China have stepped up their warnings against military intervention in Syria, with Moscow saying any such action would have "catastrophic consequences" for the region.
The US and its allies are considering launching strikes on Syria in response to deadly attacks last week.
The US said there was "undeniable" proof of a chemical attack, on Monday.
UN chemical weapons inspectors are due to start a second day of investigations in the suburbs of Damascus.
The UN team came under sniper fire as they tried to visit an area west of the city on Monday.
A spokesman for UK Prime Minister David Cameron says the UK is making contingency plans for military action in Syria.
Mr Cameron has cut short his holiday and returned to London to deal with the Syrian crisis.
Russian foreign ministry spokesman Alexander Lukashevich has called on the international community to show "prudence" over the crisis and observe international law.
"Attempts to bypass the Security Council, once again to create artificial groundless excuses for a military intervention in the region are fraught with new suffering in Syria and catastrophic consequences for other countries of the Middle East and North Africa," he said in a statement.
Meanwhile President Obama is weighing a military strike against Syria that would be of limited scope and duration, designed to serve as punishment for Syria’s use of chemical weapons and as a deterrent, while keeping the United States out of deeper involvement in that country’s civil war, according to senior administration officials.
The timing of such an attack, which would probably last no more than two days and involve sea-launched cruise missiles — or, possibly, long-range bombers — striking military targets not directly related to Syria’s chemical weapons arsenal, would be dependent on three factors: completion of an intelligence report assessing Syrian government culpability in last week’s alleged chemical attack; ongoing consultation with allies and Congress; and determination of a justification under international law ( via bbc.co.uk ).