The Syrian government has agreed to allow UN inspectors to investigate allegations of a chemical weapon attack near Damascus last week.
The UN team is to begin work on Monday. Activists say Syrian forces killed more than 300 people in several suburbs east and west of the capital on Wednesday, and numerous amateur videos have provided evidence that a chemical gas attack did indeed take place.
The Syrian foreign ministry statement broadcast on state television said an agreement to allow UN chemical weapons experts to "investigate allegations of chemical weapons use in Damascus province" had been reached on Sunday with the UN's disarmament chief, Angela Kane.
The agreement was "effective immediately", the statement added.
A spokesperson for UN Secretary General Ban Ki-moon subsequently announced that the inspectors were "preparing to conduct on-site fact-finding activities", starting on Monday.
"The secretary general notes the government of the Syrian Arab Republic affirmed that it will provide the necessary co-operation, including the observance of the cessation of hostilities at the locations related to the incident," the spokesperson added.
A top Obama administration official says there is "very little doubt" the Syrian government used used a chemical weapon on civilians, killing at least 100 people.
The official said Sunday that U.S. intelligence officials made the assessment based on "the reported number of victims, reported symptoms of those who were killed or injured and witness accounts."
The official said the White House also thinks Syrian President Bashar Assad is denying United Nations investigators immediate access to the sites of the reported chemical weapons attacks, in order to give the evidence time to degrade.
"Any belated decision by the regime to grant access to the UN team would be considered too late to be credible, including because the evidence available has been significantly corrupted as a result of the regime's persistent shelling and other intentional actions over the last five days," the official told reporters in Washington.
"Based on the reported number of victims, reported symptoms of those who were killed or injured, witness accounts, and other facts gathered by open sources, the US intelligence community, and international partners, there is very little doubt at this point that a chemical weapon was used by the Syrian regime against civilians in this incident."
Medecins Sans Frontieres (MSF) said on Saturday that three hospitals in the Damascus area had treated about 3,600 patients with "neurotoxic symptoms" early on Wednesday morning, of whom 355 died.
MSF said it could not "scientifically confirm" the use of chemical weapons, but staff at the hospitals described a large number of patients with symptoms including convulsions, pin-point pupils and breathing problems.