U.S. officials told media in America on Thursday that blood and urine samples from the victims of the suspected chemical attack last week in Douma have come out positive for chlorine and a nerve agent.
According to NBC News, the officials said that they were "confident" in the intelligence but not 100 percent sure.
They also said that the U.S. has gathered intelligence from the U.S. and other countries, including images, that indicate the Syrian government was behind the weekend attack.
U.S. President Donald Trump will be presented with the findings as he weighs options for the retaliation, an official who is familiar with the findings told NBC.
French President Emmanuel Macron said France has proof the Syrian government was behind the attack, but added that he would decide whether to intervene once all the necessary information had been gathered.
"We have proof that last week... chemical weapons were used, at least chlorine, and that they were used by the regime of Bashar al-Assad," Macron said during a television interview.
Macron said French and U.S. officials were "working together very closely, and we will have decisions to take, at the time we choose, when we consider it most useful and most effective."
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Asked if Syrian chemical weapons facilities would be the target of strikes, Macron replied: "When we decide, and we will have to verify all the information."
"We will need to take decisions in due course, when we judge it most useful and effective," Macron said.
Macron said in February this year that “France will strike” if chemical weapons are used against civilians.
German Chancellor Angela Merkel discussed the attack with Macron on Thursday, her office said.
Merkel, who spoke with Macron by telephone, told him that she was concerned that the international community's ability to enforce a ban on the use of chemical weapons was eroding.
Germany will not join any military strikes against the Syrian government in response to a poison gas attack on an opposition enclave, but supports Western efforts to show that the use of chemical weapons is unacceptable, Merkel said.
"Germany will not take part in possible - there have not been any decisions yet, I want to stress that - military action," she said after meeting Danish Prime Minister Lars Lokke Rasmussen in Berlin.
"But we support everything that is being done to show that the use of chemical weapons is not acceptable," she added.
Earlier on Thursday, Syrian government forces raised their flag over the last rebel bastion in eastern Ghouta, taking full control of the town of Douma as insurgents withdraw, Russian news agencies reported on Thursday, sealing a major victory for President Bashar al-Assad.
Eastern Ghouta had been the biggest rebel stronghold near Damascus, but insurgent groups there surrendered after a series of ferocious government assaults aided by Russia following a massive bombardment.
The Jaish al-Islam group in Douma agreed on Sunday to withdraw, hours after the suspected chemical weapons attack on the town that has raised the prospect of U.S. strikes. The Syrian government and Russia have called reports of the attack bogus.
"The raised state flag over a building in the town of Douma has heralded the control over this location and therefore over the whole of eastern Ghouta," Major-General Yuri Yevtushenko, head of the Russian Peace and Reconciliation Centre in Syria, was quoted as saying by Russian news agencies.
DPA contributed to this report