Britain will refer Syrian opposition claims that hundreds of people were killed in a chemical weapons strike near Damascus to the UN Security Council, Foreign Secretary William Hague says.
Hague said he was "deeply concerned" by the reports on Wednesday and said if they were proved they would mark a "shocking escalation".
Syria's main opposition group accused President Bashar al-Assad's regime of killing more than 650 people in a chemical weapon attack near Damascus on Wednesday.
"I am deeply concerned by reports that hundreds of people, including children, have been killed in air strikes and a chemical weapons attack on rebel-held areas near Damascus.
"These reports are uncorroborated and we are urgently seeking more information. But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria.
"Those who order the use of chemical weapons, and those who use them, should be in no doubt that we will work in every way we can to hold them to account."
Hague called on the Syrian government to allow access to the area to UN investigators already in the country probing previous claims of chemical weapons use by the regime.
"The UK will be raising this incident at the UN Security Council," he said.
