U.S. Secretary of Defense Jim Mattis warned Syria it would be "very unwise" to use gas as a weapon in its bombardment of rebel strongholds in eastern Ghouta.
Mattis said Sunday that "right now" the United States is "getting reports" that Syrian President Bashar al-Assad's forces are using chlorine gas in their three-week advance against the rebels outside the capital, Damascus, that has left nearly 1,000 dead, including 200 children.
WATCH: Syrian fighting
The U.S. defense chief acknowledged to reporters traveling with him on a trip to Oman that, "I don't have evidence that I can show you" about gas attacks, but said he was aware of "an awful lot of reports about chlorine gas use or about symptoms that could be resulting from chlorine gas."
Mattis said Syrian troops are "at best, indiscriminately" attacking and "at worst, targeting hospitals. I don't know which it is, whether they're incompetent or whether they're committing illegal acts, or both." Syria has rejected claims that it is using poison gas in its ongoing attacks on eastern Ghouta, which are occurring during the U.N. Security Council demand for a 30-day cease-fire.
The defense chief said, "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."
President Donald Trump last April ordered a missile strike against a Syrian airbase at Shayrat after the United States said it had used the facility to launch a sarin nerve gas attack on the rebel-held town of Khan Sheikhun.
Rebel fighters in eastern Ghouta have vowed not to surrender to the Syrian advance, but government forces, backed by Russian troops, have inflicted heavy damage and split the rebel-held areas into three parts.