Blood and urine samples from the site of a suspected chemical weapons attack in Syria have tested positive for chlorine gas and a nerve agent, according to U.S. officials.

MSNBC reported Thursday that the U.S. obtained samples from the attack site in Douma, a suburb of Damascus, and determined that they contained traces of chlorine gas and an unidentified nerve agent.

While officials did not identify the nerve agent as sarin, the Syrian government is believed to have used the deadly chemical weapon a number of times during the country's seven-year civil war.

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U.S. officials are confident the Syrian government was behind the attack in Douma over the weekend that left dozens of people dead, MSNBC reported.

French President Emmanuel Macron also said on Thursday that his country has "proof" that the Syrian government carried out the attack and that France would decide "in due course" whether to respond.

The revelation comes hours after the Organization for the Prohibition of Chemical Weapons announced that a fact-finding mission was en route to Syria and would begin investigating the suspected chemical attack on Saturday.

Syria and its main allies, Russia and Iran, have denied that the government used chemical weapons in the attack and have blamed the allegations on militants, who they say fabricated accounts of the strike to spark international outrage.

President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE said on Monday that he would soon decide whether to respond to the alleged chemical attack, and both France and the United Kingdom have signaled support for possible action.

Despite his claim on Monday that he would make a decision in the next "24 to 48 hours," Trump appeared to walk back that timeline on Thursday, writing on Twitter that it "could be very soon or not so soon at all!"

Trump authorized a missile strike on a Syrian air base in April 2017 in retaliation for a chemical attack in northern Syria that left more than 80 people dead. The New York Times reported on Tuesday, however, that he is weighing a more aggressive response this time around.