Jim Mattis
Jim Mattis Reuters

U.S. Defense Secretary Jim Mattis said Sunday it would be "very unwise" for the Syrian regime to use gas as a weapon against the people of Eastern Ghouta and elsewhere, AFP reports.

Mattis's warning came amid reports that Syrian President Bashar Al-Assad's forces have used chlorine gas in the rebel-held area near Damascus. He underscored his remarks by referencing President Donald Trump's strike on a Syrian airbase after an alleged chemical attack last year.

"We have made it very clear that it would be very unwise to use gas against people, civilians on any battlefield," Mattis was quoted as having told reporters accompanying him on a trip to Oman.

"I just want to reiterate that it would be very unwise for them to use weaponized gas, and I think President Trump made that very clear early in his administration."

Mattis said he was aware of "an awful lot of reports about chlorine gas use or about symptoms that could be resulting from chlorine gas," but indicated he did not have conclusive evidence.

The Hague-based Organization for the Prohibition of Chemical Weapons (OPCW) has opened an investigation into the latest reports of the use of chemical weapons in Eastern Ghouta.

The Syrian regime has repeatedly denied using chemical weapons and did so again several days ago, claiming that “terrorist groups” including the Al-Nusra Front and Islamic State (ISIS) had obtained some stocks.

The use of chlorine as a weapon is banned under international law and Russia was supposed to oversee the destruction of Syria's chemical weapons arsenal.

The fact that Assad may still have chemical weapons shows "either Russia is incompetent or in cahoots with Assad," Mattis said Sunday, according to AFP.

Syria's besieged Eastern Ghouta region is the last opposition-controlled pocket near Damascus. For nearly three weeks, regime forces have pounded it in an assault that has killed over 1,000 civilians.

Mattis also took aim at Russia, which has been propping up Assad's regime since 2015 and helping in anti-rebel operations.

Assad "could not be in power absent Russia's unfortunate veto in the UN years ago and the Russians' full-throated military support for Assad," he said.

Mattis declined to say whether the use of gas would represent some sort of trigger that would prompt a U.S. military response and would only indicate that “the president has full political maneuver room to take the decision that he believes appropriate."