The U.S. has a "very high confidence" that the Syrian government was behind a deadly chemical weapons attack in a suburb of Damascus last weekend, the White House said Friday.

"We have a very high confidence that Syria was responsible and, once again, Russia’s failure to stop them and their continued disaction on this front has been part of the problem," White House press secretary Sarah Huckabee Sanders said.

State Department spokeswoman Heather Nauert also said Friday that the U.S. has proof that the Syrian government was responsible for the attack on Douma that left dozens of civilians dead.

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"We can say that the Syrian government was behind the attack," Nauert told reporters at a press briefing.

The comments from the Trump administration officials are among the most explicit to date blaming the government of Syrian President Bashar Assad for the chemical strike.

Syria and its allies — Russia and Iran — have denied that Assad's government used chemical weapons. The Russian Defense Ministry blamed the United Kingdom on Friday for staging the attack, an accusation the U.K. dismissed as a "blatant lie."

Investigators from the Organization for the Prohibition of Chemical Weapons arrived in Syria on Thursday, and are set to begin a fact-finding mission in Douma on Saturday.

MSNBC also reported Thursday that blood and urine samples obtained by the U.S. from victims of the chemical strike tested positive for chlorine gas and a nerve agent. While the report did not identify the nerve agent, the Syrian government is believed to have used sarin on multiple occasions throughout the country's seven-year civil war.

Nauert said Friday that U.S. officials were still working to determine which chemicals were used in the Douma strike.

The dispute over the chemical attack has led to a bitter back-and-forth between Russia and the leaders of the U.S., France and Britain.

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 this week threatened a retaliatory strike on Syria, though it's unclear when — or if — such an attack will materialize.

The U.K. and France have also expressed support for a potential response to the chemical attack in Douma.

--Updated at 5:22 p.m.