Mattis’s remarks come a day after the Pentagon said it had seen “active preparations for chemical weapons use” at Shayrat Airfield, the same place struck by more than 50 cruise missiles earlier this year.
In April, President Trump ordered the strike against the base in response to what Washington claimed was a gas attack carried out by Assad’s forces. The attack likely involved the use of Sarin gas and was responsible for the deaths of more than 85 people, including children.
Mattis did not say if the Pentagon had seen activity elsewhere, but indicated that Assad’s chemical weapons program remains firmly intact despite his earlier pledges to dismantle it.
“I think that Assad’s chemical program goes far beyond one airfield,” Mattis said.
Since the United States sent a small detachment of troops into Syria in 2015, it has only become more embroiled in the six-year-old conflict.
In the country’s south, U.S. forces have sporadically bombed Iranian-backed militias in what the Pentagon has called acts of self-defense. Last week, a U.S. aircraft shot down a Syrian government jet that had bombed U.S.-backed fighters fighting the Islamic State in the militants’ de facto capital, Raqqa.
In response, Syrian ally Russia said it would consider as targets any U.S. or allies’ airplanes that flew west of the Euphrates River, which covers most of Syria outside Raqqa and other areas controlled by the Islamic State.
Despite the spate of incidents, Mattis has rejected any notion of “mission creep” and said that Washington is completely focused on fighting the Islamic State.
“We just refuse to get drawn into the Syrian civil war,” Mattis said Monday.