Syria must allow UN inspectors immediate access to investigate claims that chemical weapons were used in an attack on the outskirts of Damascus, William Hague has demanded.
The Foreign Secretary said that uncorroborated reports of toxic agents being used would mark a "shocking escalation" if they are verified and warned that those who use them "should be in no doubt that we will work in every way we can to hold them to account".
Mr Hague said: "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.
"I call on the Syrian government to allow immediate access to the area for the UN team currently investigating previous allegations of chemical weapons use. The UK will be raising this incident at the UN Security Council."
Opposition groups accused President Bashar Assad's regime of carrying out a deadly attack using rockets with poisonous gas heads.
Government officials said the claims were baseless.
A UN chemical weapons team arrived in Syria on Sunday to investigate three sites where chemical attacks previously allegedly occurred.
Shadow foreign secretary Douglas Alexander said: "The use of chemical weapons by any side in the Syria conflict deserves the categoric condemnation of the whole international community.
"These latest reports underline the need for the Syrian authorities to grant immediate and unfettered access to the team of UN chemical weapons inspectors currently on the ground.
"This access should include the sites surrounding Damascus where reports today suggest that chemical weapons may have been used."