The UK and the US have threatened a "serious response" if it emerges Syria used chemical weapons last week, Downing Street has said.
UK Prime Minister David Cameron and US President Barack Obama spoke on the phone for 40 minutes on Saturday.
Both were "gravely concerned" by "increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime".
The BBC reports the Syrian regime and opposition have accused each other over the attacks.
Rebels and opposition activists accuse forces supporting President Bashar al-Assad of carrying out chemical attacks around Damascus on 21 August, while state TV accuses the rebels.
"The UN Security Council has called for immediate access for UN investigators on the ground in Damascus," the Downing Street statement said.
"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.
Mr Cameron also spoke to Canadian Prime Minister Stephen Harper who was in agreement that "the international community must respond appropriately", Downing Street said.
Doctors believe patients hit by chemical attack
International medical aid agency Medecins Sans Frontieres says it is quite clear from the evidence that Syrian doctors have seen that many people were exposed to a neurotoxin in a Damascus suburb.
The Syrian Government denies using chemical weapons in the suburb of Ghouta on Wednesday and blames opposition forces for the apparent poison gas attack.
Medicins Sans Frontieres says it has a strong and reliable connection with medical teams and hospitals in Syria and the information it has gleaned from them suggests 3600 patients were admitted with neurotoxic symptoms in just three hours on the day of the attack.
It says 355 of those patients have died. One doctor treating the victims also died after succumbing to the effects of the toxin, the BBC reports.
The patients were suffering convulsions, pinpoint pupils and respiratory distress.
MSF says it cannot scientifically confirm the cause of the symptoms, however all evidence points to a chemical attack.
The statement was published as the United Nations' top disarmament official arrived in Damascus to press the Syrian Government to give investigators access to the site of the attack.
Angela Kane did not talk to reporters as she arrived in Syria and went directly to join the 20 chemical weapons inspectors already in the country.
Her job is to press the Syrian Government to allow the team of experts to carry out an immediate, thorough investigation of the latest alleged attack.
There has so far been no sign that it is prepared to do so, despite international calls for cooperation.
In an apparent attempt to strengthen the Government's denials of responsibility, state television has reported that soldiers have found chemical materials that had been used by rebels, and in some cases they started to suffocate.
