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 on Tuesday said Syrian President Bashar Assad has been warned against using chemical weapons on the war-torn country’s last major rebel stronghold, but he declined to say whether the U.S. would take military action if such weapons were used.

“The first time around he lost 17 percent of his pointy nosed Air Force airplanes," Mattis told reporters at the Pentagon, referring to the U.S. strike on a Syrian airfield in April 2017 following a chemical weapons attack by the Assad government on residents in the northern Idlib province.

ADVERTISEMENT

"He’s been warned," Mattis added. "And so we'll see if he's wised up,”

The U.S. continues to monitor heightened tensions in Syria's Idlib province, which last week was hit with roughly 30 airstrikes by Russia, a Syrian ally. After the Russian strike, the Trump administration warned Assad against using chemical weapons.

Asked if the U.S. would respond with military force if Assad uses chlorine gas on civilians in Idlib, Mattis replied: “I am not going to give that clarity.”

In addition to the April 2017 U.S. strike on Syria, President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE also ordered “precision strikes” against Syria this past April in retaliation for a separate chemical weapons attack by Assad’s forces.

Watergate reporter Bob Woodward's latest book notes that Trump had pushed for Assad to be assassinated in a conversation with Mattis after the April 2017 chemical attack.

Woodward wrote that Trump urged Mattis that the U.S. should “f---ing kill” Assad.

Mattis reportedly went along with the president’s demands during the phone call, but immediately told aides after hanging up that they would take a “much more measured” approach.

Trump has since called the book “fiction,” and said the assassination of Assad was "never even discussed."

Mattis has also denied the account.

Asked Tuesday if Woodward had run his quotes by him before publication, Mattis declined to discuss the matter, saying it was inappropriate to address such issues on the anniversary of 9/11.

“Of all days, this is not a day to discuss politics,” he replied.