The United Nations has demanded that Syria give its chemical weapons experts immediate access to rebel suburbs in Damascus.
Syrian rebels say more than 1000 people were killed and the government carried out the attack; the government denies this.
France says that if Syria is proved to have used chemical weapons against its own people, it could merit an international "reaction with force".
Turkey's foreign minister Ahmed Davutoglu, described it as a crime against humanity, saying all options are on the table in response.
But Russia, said the attack was a provocation by the rebels to draw in Western military assistance.
However, there is no sign that the UN team will be allowed to investigate.
The US State Department said it is unable to conclusively say chemical weapons were used and American intelligence agencies would urgently gather information on the claims.