U.N. Secretary-General Ban Ki-moon says U.N. personnel will begin inspecting the site of a major suspected chemical attack near Damascus on Monday.
In a statement released by his spokesman on Sunday, Ban said the Syrian government agreed to "provide the necessary cooperation" for the U.N. team to investigate the August 21 incident.
Ban said that cooperation includes ceasing hostilities in the area of the attack in the rebel-held Damascus suburb of Ghouta.
A senior Obama administration official dismissed the Syrian government's offer to let the U.N. inspectors access the site, calling it "too late to be credible."
The U.S. official said there is "very little doubt" that forces loyal to Syrian President Bashar al-Assad used a chemical weapon against civilians in Ghouta.
The official said the available evidence of Wednesday's attack has been "significantly corrupted as a result of the regime's persistent shelling and other intentional actions over the last five days."
The Syrian government pledged to work with visiting U.N. disarmament envoy Angela Kane to enable the U.N. team to visit the attack site. It has denied carrying out a chemical attack and accused Syrian rebels of being responsible for using such weapons.
The U.N. inspectors led by Swedish scientist Ake Sellstrom arrived in Damascus a week ago to investigate earlier allegations of chemical attacks in the Syrian conflict.
Western powers had called on the Syrian government to give the U.N. team immediate access to the scene of Wednesday's incident, fearing any delay would enable evidence of a chemical attack to degrade or be removed.
The U.S. official said Washington and its international partners believe there is little doubt of Syrian government responsibility 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."
Syrian rebels and activists said the alleged government chemical attack killed hundreds of civilians and released footage to back up their claim, showing scores of bodies and patients writhing in agony, all with no visible wounds.
Syrian state news agency SANA accused rebels of carrying out a new chemical attack on Saturday, causing soldiers to suffocate in the Joubar suburb of Damascus.
U.S. Defense Secretary Chuck Hagel said Sunday that U.S. forces are prepared to take action against Syria, if President Barack Obama approves. Obama met with top U.S. military and national security advisors on Saturday to consider his options.
The office of French President Francois Hollande said he and British Prime Minister David Cameron agreed on Sunday to hold talks soon on how to respond to what the French leader called the Syrian government's "intolerable act."
It also quoted Hollande as saying the suspected chemical attack will not go "unpunished."
Israeli Prime Minister Benjamin Netanyahu suggested that he may take action against longtime enemy Syria, telling a Cabinet meeting that "our finger is responsible, and when needed it is also on the trigger."
He said the Ghouta attack demonstrates that what he called the "world's most dangerous regimes" must not be allowed to posses the "world's most dangerous weapons."
Syria and its allies warned against any U.S.-led military strikes.
In an interview broadcast Saturday, Syrian Information Minister Omran Zoabi told Lebanon-based al-Mayadeen television that U.S. military intervention will create a very serious fallout and a "fire that will inflame the Middle East."
Russia's foreign ministry accused some nations of trying to "impose their opinion" on the U.N. inspectors. It said those nations that suggest they could take military action against Syria should "use their common sense" and "refrain from committing a tragic mistake."
The head of the Russian parliament's International Affairs Committee posted a message on Twitter saying President Obama risks becoming a "clone" of his predecessor George W. Bush. Alexei Pushkov said Obama is moving toward what he called an "illegitimate" war in Syria in the same way that Bush did with Iraq.
Iran's Fars news agency also quoted the Iranian military's deputy chief of staff Massoud Jazayeri as saying "any crossing of Syria's red line will have severe consequences for the White House."
In Syria's latest violence, Syrian state television reported that the governor of Hama province was assassinated in a car bombing on Sunday.