William Hague has demanded immediate access for UN inspectors to determine whether Syria has used chemical weapons against rebel forces.
The Foreign Secretary said the uncorroborated reports of toxic agents being used on the outskirts of Damascus would mark a "shocking escalation" if verified.
And he warned those who use them "should be in no doubt that we will work in every way we can to hold them to account".
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."