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 hinted Friday that the United States would strike the Syrian regime again if it uses sarin gas, but added that the United States has no evidence it has used the nerve agent recently.

“We’re on the record and you all have seen how we reacted to that, so they would be ill-advised to go back to violating the chemical convention,” Mattis told reporters at an off-camera gaggle at the Pentagon, according to multiple reports.

Mattis appeared to be referencing the April cruise missile strike against a Syrian airfield after the regime of Syrian President Bashar Assad used sarin on civilians in the town of Khan Shaykhun.

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Reports out of Syria have indicated several chlorine gas attacks in recent weeks.

On Friday, Mattis said it was clear Syria has weaponized chlorine, but added that he doesn’t have evidence of sarin use.

“We’re even more concerned about the possibility of sarin use,” he said. “I don’t have the evidence. What I’m saying is groups on the ground, [nongovernmental organizations] NGOs, fighters on the ground said that sarin has been used. So we are looking for evidence. I don’t have evidence.”

Assad has repeatedly denied using chemical weapons. He agreed to give up his stockpile of chemical weapons as part of a 2013 deal after a sarin attack in Ghouta killed about 1,400 people.

Mattis said the United States is looking for evidence of sarin use because “clearly we are dealing with the Assad regime that has used denial and deceit to hide it.”

The United States has in recent weeks stepped up its pressure on Assad over the use of chemical weapons.

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Last month, Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE said reports of chlorine gas attacks in East Ghouta raised “serious concerns.” He blamed Russia, as a guarantor of the 2013 disarmament agreement, for failing to enforce it. Assad did not have to relinquish chlorine as part of the agreement, though its use as a weapon violates the Chemical Weapons Convention.

“Russia has not lived up to these commitments,” Tillerson said. “Since April 2014, there has been mounting evidence that Syria continues to illicitly possess chemical weapons and use them against its own people.”

On Thursday, unnamed U.S. officials told The Associated Press that they believe Assad is working on “new kinds” of chemical weapons to possibly improve their military capability or evade international detection.