Syrian opposition leaders have accused forces loyal to Syrian President Bashar al-Assad of killing hundreds of people in attacks using "poisonous gas" on rebel-held areas of Damascus.
Estimates of reported death tolls from human rights activists varied widely. George Sabra of the exiled Syrian National Coalition estimated the number of casualties at 1,300. His claim at a news conference in Istanbul on Wednesday could not be independently verified.
The Syrian government denied using chemical weapons in Wednesday's military operations. Syrian activists claimed Syrian troops unleashed an artillery and rocket barrage against several Damascus suburbs on Wednesday that allegedly included some chemical elements.
The activists posted videos online showing scores of bodies of adults and children laid out on the floor of makeshift clinics with no visible signs of injuries.
The Syrian government, however, said the opposition is attempting to disrupt a mission by U.N. personnel who are in Syria to investigate previous allegations that chemical weapons were used in Syria's civil war earlier this year.
Britain, France and the Arab League called for the U.N. team led by Swedish scientist Ake Sellstrom to visit the sites of Wednesday's alleged chemical attacks. Sellstrom said the incidents should be "looked into."
British Foreign Secretary William Hague said he is "deeply concerned" about the reported use of chemical weapons. He said that if verified, they would represent a "shocking escalation" of the Syrian conflict. Hague also 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 for 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 mandate of Sellstrom's U.N. inspection team is limited to establishing whether chemical weapons - including sarin and other toxic nerve agents - were used, not who used them.
The Syrian government also has restricted the mission to investigating several specific incidents including a March attack in the Aleppo suburb of Khan al-Assal.