The Assad regime has twice been targeted by US air and missile strikes after previous alleged chemical attacks, and US officials have in recent days said additional action would follow if Assad were to use the banned weapons in rebel-held Idlib.

"In Idlib, we're watching very closely what the Assad regime, aided and abetted by the Iranians and the Russians are up to there," Mattis told Pentagon reporters.

Pointing to a US missile strike on a Syrian air base in April 2017 after Assad's jets launched an alleged chemical attack in Idlib, Mattis noted that the Syrian leader suffered massive losses to his air force.

"The first time around, he lost 17 percent of his pointy-nosed air force airplanes," Mattis said, referring to the unusual nose cones on Russian-built jets.

"He's been warned, and so we'll see if he's wised up," he added.

A second strike, in April this year, saw British and French militaries assist the US-led mission.

Idlib province and adjacent rural areas form the largest piece of territory still held by Syria's beleaguered rebels, worn down by a succession of government victories in recent months.

Assad has now set his sights on Idlib, and his forces have stepped up bombardment of the densely populated province since the beginning of the month.