French President Francois Hollande told his US counterpart Barack Obama that "everything was consistent" with the conclusion that Damascus was behind the attack.
"The two presidents agreed to stay in close contact to arrive at a joint response to this unprecedented aggression," the French leader's office said.
And British Foreign Secretary William Hague warned any evidence of a chemical attack may have been destroyed. "The fact is that much of the evidence could have been destroyed by that artillery bombardment," he said.
However, French Prime Minister Jean-Marc Ayrault implied that a punitive strike on Syria was not imminent, in an interview with France 2 television.
"Once this (UN) investigation has ended, we await a firm decision, a clear decision, from the international community. The (UN) Security Council will meet," he said.
Syria's opposition says more than 1,300 people died when regime forces unleashed chemical weapons against rebel-held towns east and southwest of Damascus last Wednesday, while Doctors Without Borders said 355 people had died of "neurotoxic" symptoms.
Damascus has strongly denied it carried out such an attack, instead blaming the rebels.
With the drums of a wider war beating, Syria's ally Moscow bluntly warned the West that military action against President Bashar al-Assad's regime would be a "tragic mistake".
"We strongly urge those who, by attempting to impose their own results on the UN experts, are raising the possibility of a military operation in Syria to use their common sense and refrain from committing a tragic mistake," foreign ministry spokesman Alexander Kasevich's said in a statement.
Syria's foreign ministry said that visiting UN disarmament envoy Angela Kane on Sunday struck the accord with the Syrian government for a probe.
The United Nations said the investigation would begin as early as Monday, stressing that the rebels and government were responsible for the safety of the UN inspectors on the ground and that a local ceasefire had been agreed.
US officials said Obama, who held crisis talks Saturday with top security aides, would make an "informed decision" about how to respond to an "indiscriminate" chemical weapons attack.
Washington had noted that Syria had offered to let UN inspectors view the site of the alleged attack, but suggested it was too little, too late, said one official, speaking on condition of anonymity.
"At this juncture, the belated decision by the regime to grant access to the UN team is 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 said.
If confirmed, the attack would mark the deadliest use of chemical agents since late Iraqi dictator Saddam Hussein gassed Iranian troops and Kurdish rebels in the 1980s.
US Defence Secretary Chuck Hagel said earlier Sunday the US military was "prepared to exercise whatever option" against Syria but intelligence was still being evaluated.
On a visit to Malaysia, Hagel said the US defence department had prepared "options for all contingencies" at Obama's request.
On Saturday, Obama held a rare meeting with his top aides and discussed Syria by phone with British Prime Minister David Cameron.
Cameron's office said they two leaders agreed the use of chemical weapons would "merit a serious response" -- echoing the French calls.
Obama had said a year ago that the use of chemical weapons by Assad's forces was a "red line" that could trigger Western intervention.
On Sunday, a strident warning came from Washington's archfoe Iran.
"If the United States crosses this red line, there will be harsh consequences for the White House," armed forces deputy chief of staff Massoud Jazayeri said, without elaborating.
The Arab League is to meet on Tuesday to discuss the alleged use of chemical weapons in Syria, the bloc's deputy chief Ahmed Ben Helli said.
In Israel, President Shimon Peres called for international efforts to "take out" chemical weapons in Syria as Prime Minister Benjamin Netanyahu said Israel would pull the "trigger" if needed to protect its people.
More than 100,000 people have been killed in Syria since an uprising against Assad's rule flared in March 2011, the UN says.
In the latest eruption of violence, the governor of Hama province in central Syria was killed in a car bombing on Sunday, state television reported, in an attack it blamed on rebels.