Secretary of State Mike Pompeo says Syrian government forces used chlorine during an attack in May this year.

The United States has concluded that Syrian President Bashar al-Assad‘s forces used chemical weapons in an attack in May, Secretary of State Mike Pompeo has said, vowing a response.

The Assad regime used chlorine on May 19 as part of its deadly offensive in the last rebel stronghold of Idlib, Pompeo said on Thursday at a news conference in New York, where he has been attending the United Nations General Assembly.

“The Assad regime is responsible for innumerable atrocities some of which rise to the level of war crimes and crimes against humanity,” Pompeo told reporters.

“The US will not allow these attacks to go unchallenged nor will we tolerate those who choose to conceal these atrocities.”

Pompeo added that the US would continue to pressure the Assad regime to end the violence and participate in the United Nations-led political process.

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The US said in May it had received numerous reports that appeared consistent with chemical exposure after an attack by Syrian government forces in northwest Syria, but it had made no definitive conclusion as to whether they used chemical weapons.

International investigators say that the Russian-backed forces of Assad have repeatedly used chemical weapons against civilian targets in its brutal quest to end the civil war.

Earlier this month, the UN said the US-led coalition forces battling the Islamic State of Iraq and the Levant (ISIL or ISIS) in Syria may have committed war crimes.

The Syrian civil war, now in its ninth year, has claimed hundreds of thousands of lives and forced 13 million people from their homes.

‘This is different’

US President Donald Trump‘s administration has twice bombed Syria over Assad’s suspected use of chemical weapons, in April 2017 and April 2018.

The US, UK and France launched air raids 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.

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Assad launched an offensive at the end of April this year on Idlib and parts of adjacent provinces, saying rebel 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, declining to say what the US response could be.

Former President Barack Obama had called the use of chemical weapons a red line, but declined military retaliation.

Pompeo said on Thursday that Washington had also added sanctions on two Russian entities for providing fuel to the Syrian government.