'It's a violation of the United Nations Security Council resolutions, and it's going to have to be taken up diplomatically, and they'd be ill advised to try to use any again,' Mattis said alongside Israeli Defense Minister Avigdor Lieberman in Tel Aviv on April 21.
'We have 100 percent information that the Assad regime used chemical weapons against rebels,' Lieberman said, referring to Syrian President Bashar al-Assad.
Damascus says it turned over all chemical weapons stockpiles in 2013.
The comments come after a suspected chemical air strike by Assad's forces killed more than 80 people in the rebel-held town of Khan Sheikhoun on April 4.
Damascus and Moscow, Assad's main backer in the six-year Syrian conflict, have claimed that the attack was staged or that toxic gas was released when government air strikes hit a rebel weapons depot, assertions that Western governments reject.
Based on reporting by AFP, AP, and Reuters