The U.S. strongly suspects the regime of Syrian President Bashar Assad continues to use sarin nerve gas against his own people based on various reports from inside Syria, despite its promise to give up its stockpiles of deadly chemical weapons.

Speaking to reporters at the Pentagon Friday, Defense Secretary Jim Mattis hinted if the U.S. finds hard evidence to back up the claims, the response could be another U.S. military strike.

In April of last year, the U.S. launched a volley of cruise missiles against a Syrian airfield after confirming nerve gas use against civilians in the Syrian town of Khan Shaykhun.

“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 said.

Multiple reports claim that Syria continues to use chlorine gas as a weapon. Mattis said the U.S. is looking into reports that deadly nerve agents that Syria claims it eliminated in 2013 have shown up in attacks against opposition forces and civilians.

“We are even more concerned about the possibility of sarin use, the likelihood of sarin use, and we are looking for the evidence,” Mattis said, being careful to say the reports are still unconfirmed.

“I don’t have the evidence. What I’m saying is groups on the ground, NGOs, fighters on the ground said that sarin has been used. So we are looking for evidence. I don’t have evidence.”

Last month, Secretary of State Rex Tillerson said regime attacks in East Ghouta raised “serious concerns” that Assad is continuing to use chemical weapons, and blamed Russia who is supposed to be the guarantor of the 2013 agreement under which Syria’s entire chemical weapons stockpile would be destroyed.

“Russia has not lived up to these commitments,” Tillerson said in a speech Jan 23. “Since April 2014, there has been mounting evidence that Syria continues to illicitly possess chemical weapons and use them against its own people.

“There is simply no denying that Russia, by shielding its Syrian ally, has breached its commitments to the United States as a framework guarantor,” Tillerson said.