Pompeo was speaking at a news conference in New York, where he has been attending the UN general assembly. "The Assad regime is responsible for innumerable atrocities some of which rise to the level of war crimes and crimes against humanity,” the US secretary of state said.

The United States has concluded that the government of Bashar al-Assad used chlorine as a chemical weapon in May, marking the first confirmed violation of the ban on chemical weapons since Donald Trump authorized airstrikes in 2018 over Syria’s use of poison gas.

The US said in May it had received numerous reports that appeared consistent with chemical exposure after an attack by Syrian government forces on Idlib province in north-west 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.