It also ridiculed comments by President Bashar al-Assad in an interview with AFP to the effect that the alleged attack was fabricated by the United States to justify an American military strike.
"Sadly, it's vintage Assad. It is an attempt by him to throw up false flags, create confusion," said department spokesman Mark Toner, alluding to what Assad said in the interview Wednesday. It was Assad's first since the alleged April 4 chemical weapons attack prompted a US cruise missile strike on a Syrian air base.
"Frankly, it's a tactic we've seen on Russia's part as well in the past," Toner told a daily press briefing.
Echoing charges by Defense Secretary James Mattis and Secretary of State Rex Tillerson, Toner said there can be little doubt that the chemical weapons attack in Idlib province was carried out by Syrian government forces.
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"It wasn't only a violation of the laws of war but it was a -- we believe, a war crime," Toner said.
Tillerson did raise the prospect of criminal proceedings at some point over the attack, including against Assad himself, but warned there were be major legal obstacles to this.