Syrian activists say at least 100 people have been killed in a government offensive near the capital, Damascus, where forces loyal to President Bashar al-Assad are alleged to have used chemical weapons.
Syria quickly denied allegations of a chemical attack. State media reported military activity in the area, saying that government forces had killed "terrorists" - a term used to refer to rebels opposed to Mr. Assad.
The Britain-based Syrian Observatory for Human Rights said the military fired rockets at sites in Eastern Ghouta and Western Ghouta. It cited activists in the area who said "poisonous gasses" were used.
Britian and the Arab League have called on Syria to allow U.N. invesigators already in the country to examine sites of the alleged chemical attacks.
British Foreign Secretary William Hague said he was "deeply concerned" about the reported chemical attacks, and said if proven true, they represent a "shocking escalation." He said Britain will refer the matter to the U.N. Security Council.
Khaled Saleh, a spokesman for the main opposition Syrian National Coalition, strongly criticized the reported attacks, calling the situation in the area "extremely dire."
"This really is a slap in the face of humanity, in the face of the U.N., in the face of the Friends of Syria group, it's even a slap in the face of those regimes that support the Assad regime in killing the Syrian people. What we want is from those inspectors to come in and see the people that were killed in the country side of Damascus. We want them to look at the victims, we want them to investigate who used those chemical weapons."
The violence comes as United Nations chemical weapons inspectors carry out a two-week mission in Syria to investigate earlier incidents in which the Syrian government and rebels accused each other of carrying out chemical attacks.
The inspectors will try to establish only whether chemical weapons - including sarin and other toxic nerve agents - were used, not who used them.
The mission will be limited to investigating the alleged use of chemical weapons in three areas, including a March attack in the Aleppo suburb of Khan al-Assal.