The officials briefed reporters on condition of anonymity.
"We will continue to seek accountability through all available diplomatic mechanisms, including the United Nations Security Council and the Organization for the Prohibition of Chemical Weapons, for the confirmed use of chemical weapons by any party", the State Department said in a statement. Nauert said this is the third reported incident in east Ghouta in the last 30 days.
A USA official said Syrian President Bashar al-Assad's forces may be developing new types of chemical weapons, which could reach as far as the U.S. unless the global community steps up its pressure on Assad's regime.
President Donald Trump has not ruled out additional military action to deter attacks or punish Assad, administration officials said Thursday, although they did not suggest any action was imminent.
The United States is "extremely concerned" about reports that Syrian forces recently used chlorine gas in an attack on the eastern Ghouta area, which caused more than a dozen people to fall ill.
"We think that they did not carry out what they said they would do back when in the previous administration when they were caught using it".
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Tillerson said at a conference last week in Paris that Moscow "ultimately bears responsibility for the victims" of chemical weapons attacks in Syria since it became militarily involved in 2015.
But with Russian and Iranian influence growing in Syria and posing a direct threat to U.S. foreign policy interests, it's possible that the Trump administration may look to make a statement that it's not buying Russia's excuses anymore.
Chemical attacks aren't uncommon in parts of Syria. The accounts followed a suspected attack in late January near Damascus that activists and rescue teams said affected almost 20 civilians.
"It will spread if we don't do something", the official warned.
U.S. Secretary of State Rex Tillerson called for "patience" on the Syrian leader's departure - the clearest indication yet of an acknowledgment that Russian Federation and Iran have bolstered Assad and that he is unlikely to leave power immediately.
Mattis said Syria has likely used weaponized chlorine gas repeatedly.