At least 100 people have been killed in what Syrian activists claim is a poison gas attack during a fierce government offensive near Damascus.
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.
Rami Abdul-Rahman from the British-based Syrian Observatory for Human Rights said he has documented at least 100 deaths from the attack, adding it was not clear whether the victims died from shelling or toxic gas.
Mohammed Saeed, an activist in the area, said hundreds of dead and injured people were taken to six makeshift hospitals in the eastern suburbs.
“This is a massacre by chemical weapons,” said Mr Saeed. “The visit by the UN team is a joke. Bashar is using the weapons and telling the world that he does not care.”
UK Foreign Secretary William Hague said the reports of toxic agents being used would mark a “shocking escalation” if they are verified.
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.”