A survivor from what activists say is gas attack vomits inside a mosque in the Duma neighbourhood of Damascus August 21. Photo: Reuters
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 phone for 40 minutes on Saturday, Cameron's office said.
Both were "gravely concerned" by "increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime".
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 Cameron and 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.
"The president has directed the intelligence community to gather facts and evidence so that we can determine what occurred in Syria," a White House official told AFP news agency.
"We have a range of options available, and we are going to act very deliberately so that we're making decisions consistent with our national interest as well as our assessment of what can advance our objectives in Syria."
On Saturday, US Defence Secretary Chuck Hagel said the Pentagon had presented options to Obama and he suggested that US forces were being moved ahead of a possible decision on taking action against Syria.
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.
Our correspondent added that some British Conservative MPs had made it very clear they did not want Cameron to commit himself to any form of military intervention in Syria without consulting MPs first.
Meanwhile, medical charity Medecins Sans Frontieres (MSF) says hospitals it supports in Syria have treated about 3,600 patients with "neurotoxic symptoms", of whom 355 have died.
MSF, which says it cannot "scientifically confirm" the use of chemical weapons, said the patients had arrived in three hospitals in the Damascus area on Wednesday.
It says staff at the hospitals described a large number of patients arriving in the space of less than three hours with symptoms including convulsions, dilated 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.
A team of UN weapons inspectors has been in Syria since 18 August to inspect three sites, but the government has not yet said whether it will allow them to expand their visit.
The UN says more than 100,000 people have been killed since the uprising against President Assad began more than two years ago.