The UK and the US have threatened a "serious response" if it emerges Syria used chemical weapons last week.
Prime Minister David Cameron and President Barack Obama spoke on the telephone for 40 minutes on Saturday.
Both were "gravely concerned" by the "increasing signs that this was a[n]... attack carried out by the Syrian regime", Cameron's office said.
But intervention would have serious consequences and the US case was weak, the Syrian information minister warned.
In an interview with Lebanese TV, Omran Zoabi said: "If the US leads a military intervention, this will have dangerous consequences. It will bring chaos and the region will burn."
The Syrian government has denied any use of chemical weapons, blaming rebel fighters instead.
State television reported on Saturday that soldiers had found chemical agents in tunnels used by the rebels to the east of Damascus.
It broadcast images of gas masks and plastic containers, but nothing to support official statements that soldiers had "suffered from cases of suffocation" when rebels used poison gas "as a last resort" after government forces made "big gains" in the suburb of Jobar.
Opposition activists accuse forces supporting President Bashar al-Assad of killing between 500 and more than 1,000 civilians in several suburbs east and west of capital in the early hours of Wednesday morning.
They want the areas inspected by UN chemical weapons experts who are already in Damascus to investigate other suspected attacks.
Out of the chaos and confusion of the past few days, several things have emerged clearly.
Even the regime itself and its closest allies, Russia and Iran, do not dispute that chemical weapons were used in the suburbs of Damascus on Wednesday. The evidence from a huge flow of distressing amateur video is too massive to dismiss.
Both Moscow and Tehran have said they are urging the Syrian authorities to co-operate with the UN chemical weapons inspectors already in Damascus, and the Iranian foreign minister has quoted his Syrian counterpart as saying the government is preparing the conditions for a site visit.
With combat continuing in the affected areas, there is clearly scope for prevarication and delay, although Western patience is short.
But at least an appearance of regime willingness to co-operate may for the moment let the US and its allies off the hook.
For one of the other elements that has become clearer than ever in the past few days is the great reluctance of US President Barack Obama and others to plunge into an embroilment that would be hard to get out of, and which would carry a very high risk of aggravating the situation even further.
The UN's disarmament chief, Angela Kane, arrived in Damascus on Saturday to press the authorities for access.
Iran's Irna state news agency reported that Syrian Foreign Minister Walid Muallem had told his Iranian counterpart that Damascus was "co-operating" with the UN experts and "preparing the opportunity for them to visit areas which have been attacked chemically by terrorist groups".
"The UN Security Council has called for immediate access for UN investigators on the ground in Damascus," Downing Street said in a statement.