US Secretary of State Mike Pompeo said on Thursday that the United States had concluded the regime of Bashar al-Assad in Syria had used chlorine as a chemical weapon in an attack in May during a battle in Idlib.
"The Assad regime is responsible for innumerable atrocities some of which rise to the level of war crimes and crimes against humanity," Pompeo told a news conference in New York, where he has been attending the United Nations General Assembly.
"Today I am announcing that the United States has concluded that the Assad regime used chlorine as a chemical weapon on May 19," Pompeo said.
The United States said in May it had received numerous reports that appeared consistent with chemical exposure after an attack by Syrian regime forces in northwest Syria, but it had made no definitive conclusion as to whether they used chemical weapons.
The Trump administration has twice bombed Syria over Assad's suspected use of chemical weapons, in April 2017 and April 2018.
The United States, Britain and France launched air strikes in April 2018 against what they described as three Syrian chemical weapons targets in retaliation for a suspected gas attack that killed scores of people in a Damascus suburb earlier that month.
Assad launched an offensive at the end of April this year on Idlib province and parts of adjacent provinces, saying opposition fighters had broken a truce.
"This is different in some sense because it was chlorine... but know that President Trump has been pretty vigorous in protecting the world from the use of chemical weapons," Pompeo said, said declining to say what the US response could be.
Pompeo said Washington had also added sanctions on two Russian entities for providing fuel to the Syrian government.
The US Treasury Department on Thursday imposed sanctions on a firm it said was participating in a scheme to avoid US sanctions while helping provide jet fuel to Russian forces in Syria.
The Treasury said the newly sanctioned firm, Maritime Assistance LLC, was operating as a front company for OJSC Sovfracht, a company the United States had previously sanctioned in relation to operations in Ukraine.
The Treasury also targeted three individuals it said were tied to Sovfracht, freezing any assets they may hold in the United States and barring Americans from dealing with them.
Five ships were also designated as "blocked property" of previously sanctioned Russian firm Transpetrochart, which Washington alleges provides support to Sovfracht.
The United States accuses Sovfracht of being behind a sanctions-evasion conspiracy to make payments and facilitate the transfer of supplies of jet fuel to Russian forces operating in Syria in support of Assad.