Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE says Syrian President Bashar Assad is lying when he claims his nation doesn't have chemical weapons.

Assad has denied being behind a chemical attack on his own people earlier this month, insisting in an interview that Syria doesn't have chemical weapons. But Mattis says there's no question he does.

“There can be no doubt in the international community’s mind that Syria has retained chemical weapons in violation of its agreement and its statement that it has removed them all,” Mattis said Friday in Israel, according to The Associated Press.

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“I can say authoritatively they have retained some, it’s a violation of the United Nations Security Council resolutions and it’s going to have to be taken up diplomatically and the would be ill advised to try to use any again, we made that very clear with our strike.”

President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE launched a missile strike against the Syrian air base where officials say Assad staged the chemical attack that killed more than 90 civilians.

Speaking alongside Mattis, Israeli Defense Minister Avigdor Lieberman said Israel has “100 percent information that [the] Assad regime used chemical weapons against rebels.”

Senior Israeli defense officials said Wednesday that Assad’s military is believed to have as much as three tons of chemical weapons despite a 2013 agreement ordering the country to dispose of its stockpiles.