The United States has proof at "a very high level of confidence" that the Syrian government carried out the recent chemical weapons attack in Douma but is still working to identify the mix of chemicals used, the State Department said on Friday.
"I'm not going to say which day we absolutely knew that there was proof. The attack took place on Saturday, we know for a fact that it was a chemical weapon," spokeswoman Heather Nauert said.
"We know that there are only certain countries like Syria that have delivery mechanisms and have those types of weapons."
Asked whether she could say that the United States has proof that Assad's regime was behind the strike, Nauert said: "Yes."
International experts from the Organisation for the Prohibition of Chemical Weapons were due in Syria over the weekend to inspect the site where dozens of civilians reportedly died in a gas attack.
But the OPCW is not mandated to ascribe blame to any party, simply to establish facts like the kind of agent that was used, how it was delivered and how many people it killed or wounded.
Nauert said that the US administration was not relying on the OPCW findings but had sources of its own.
She acknowledged that some commentators have asked why the United States has not make its "intelligence information" public if it has any, but said "a lot of this stuff is classified right now."
Nauert said a team from the Organization for the Prohibition of Chemical Weapons would arrive in Syria on Saturday to collect evidence.
French President Emmanuel Macron and British Prime Minister Theresa May agreed to continue working on the international response to the suspected chemical attack in Douma.