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", Mr 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.
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".
'Gathering facts and evidence'
"The UN Security Council has called for immediate access for UN investigators on the ground in Damascus," Downing Street said in a statement.
"The fact that President Assad has failed to co-operate with the UN suggests that the regime has something to hide."
It said Mr Cameron and Mr Obama had "reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options".
The statement said the two men had agreed it was "vital that the world upholds the prohibition on the use of chemical weapons and deters further outrages".
They would keep in "close contact", it added.
The US president earlier convened his National Security Council to discuss options on Syria.
US Defence Secretary Chuck Hagel said the American military, which is repositioning naval forces in the Mediterranean, was ready to act.
"President Obama has asked the defence department to prepare options for all contingencies. We have done that and we are prepared to exercise whatever option - if he decides to employ one of those options," he told reporters in Kuala Lumpur.
BBC political correspondent Iain Watson said he understood that the "serious response" mentioned in the UK statement would not include "boots on the ground".
But a range of other options was not ruled out, he said, potentially including air strikes.
On Sunday, Iran's deputy armed forces chief, Massoud Jazayeri, warned the US against crossing the "red line" on Syria, saying it would have "severe consequences", according to the Fars news agency.
The medical charity Medecins Sans Frontieres (MSF) has said three hospitals it supports in the Damascus area had treated about 3,600 patients with "neurotoxic symptoms" early on Wednesday morning, of whom 355 have died.
While MSF said it could not "scientifically confirm" the use of chemical weapons, staff at the hospitals described a large number of patients arriving in the space of less than three hours with symptoms including convulsions, pinpoint pupils and breathing problems.
MSF director of operations Bart Janssens said the symptoms - as well as the "massive influx of patients in a short period of time" - strongly suggested mass exposure to a neurotoxic agent.
The Syrian Observatory for Human Rights, a UK-based activist group, meanwhile said it had documented the deaths of 322 people, including 82 women and 54 children.
On Sunday morning, several suburbs of Damascus reportedly came under heavy shell and mortar fire, as government forces continued their offensive on rebel strongholds.
At least 114 people were killed across the country on Saturday, including 33 people in the capital, according to the Local Co-ordination Committees, an opposition activist network.
The UN says more than 100,000 people have been killed since the uprising against President Assad began more than two years ago.
'Chemical attack': What we know
- 01:15: 21 August (10:15 GMT 20 Aug): Facebook pages of Syrian opposition report heavy fighting in rebel-held eastern districts of the Ghouta, the agricultural belt around Damascus
- 02:45: Opposition posts Facebook report of "chemical shelling" in Ein Tarma area of the Ghouta
- 02:47: Second opposition report says chemical weapons used in Zamalka area of the Ghouta
- Unverified video footage shows people being treated on pavements in the dark and in a makeshift hospital
- Reports say chemical weapons were used in Ghouta towns of Irbin, Jobar, Zamalka and Ein Tarma as well as in Muadhamiya to the west, but this is not confirmed
- Syrian government acknowledges military offensive in the Ghouta but denies chemical weapons use