Washington (CNN) US Defense Secretary James Mattis on Sunday warned Syria against using chemical weapons, implying it would prompt military consequences, as it did last year when President Donald Trump ordered a strike on a Syrian air base.

"I just want to reiterate it would be very unwise for them to use weaponized gas," Mattis told reporters while en route to Oman. "And I think President Trump made that very clear early in his administration."

Mattis blamed Russia for Syria's chemical weapons stockpile, which Russia said it would help eliminate as part of an agreement in 2013, but which the US and international observers said Syria used on civilians last year.

"Russia was the framework guarantor that (Syrian dictator Bashar al Assad) would get rid of all of it," Mattis said. "Again, either Russia is incompetent or in cahoots with Assad."

Mattis also expressed skepticism at the much vaunted roll-out of supposedly new, high tech Russian missiles, which Russian President Vladimir Putin has claimed are "invincible" and have unlimited range.

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