The United Nations deputy secretary-general says any use of chemical weapons in Syria would represent a "serious escalation" in the fighting there and would have "grave human consequences."
Jan Eliasson made his comments Wednesday afternoon after an urgent U.N. Security Council meeting on Syria.
Eliasson said he hopes U.N. inspectors now in Syria will gain access to the sites near Damascus where the opposition has accused government forces of killing many civilians in chemical weapon attacks on Wednesday.
He said U.N. officials are in contact with Syria's government about gaining such access, and he noted that the situation is "very dramatic."
The Arab League and Western powers including the U.S. have urged Syria's government to allow U.N. inspectors to immediately visit the sites of Wednesday's alleged attacks.
Syria's government has denied using chemical weapons in its latest assault on rebel-held Damascus suburbs. It accuses the opposition of trying to distract U.N. inspectors who arrived in Syria this week to investigate government claims that rebels used chemical weapons earlier this year.
Russia, a key Assad ally, accused the opposition of committing a "premeditated provocation" by making claims about mass casualties from a government chemical attack soon after the arrival of the U.N. inspectors.
Syrian opposition reports of the death toll from Wednesday's attacks varied widely. Opposition leader George Sabra of the exiled Syrian National Coalition told a news conference in Istanbul the number of those killed is as high as 1,300. His claim could not be independently verified.
Syrian activists said government troops unleashed an artillery and rocket barrage against several Damascus suburbs, with some of the weapons allegedly containing chemical elements. They 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 White House said it is "deeply concerned" by the reports and called for those responsible for using chemical weapons to be held accountable. It said the Syrian government must grant the U.N. investigators "immediate access to witnesses and affected individuals," and allow them to "examine and collect physical evidence without any interference or manipulation."
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 countryside of Damascus. We want them to look at the victims, we want them to investigate who used those chemical weapons."
The mandate of the 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.