Washington (CNN) Secretary of Defense James Mattis has warned the Syrian regime against the use of chemical weapons, telling reporters Friday "you have all seen how we reacted to that, so they'd be ill advised to go back to violating the chemical convention."

Mattis was referring to the April 2017 military strike when the US launched Tomahawk missiles against Syria in response to a chemical weapons attack that killed dozens of civilians.

Asked if the Syrian regime was using chlorine weapons Mattis said he believed they had done so "repeatedly" but said that the US is "even more concerned about the possibility of Sarin use, the likelihood of Sarin use, and we're looking for the evidence."

Mattis said he does not believe that Syria has kept to the 2013 agreement brokered by the US and Russia that required the Assad regime to relinquish their chemical weapons.

"We think that they did not carry out what they said they would do back when in the previous administration when they were caught using it. Obviously, they didn't, because they used it again during our administration, and that gives us a lot of reason to suspect them, and now we have other reports from the battlefield, from people who claim it's been used, we do not have evidence of it, but we are not refuting them, we are looking for evidence of it, since clearly we are dealing with the Assad regime that has used denial and deceit to hide their outlaw actions."

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