Assad 'has 'been warned' over chemical weapons use: Mattis

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

Pointing to a US missile strike on a Syrian air base in April 2017 after Assad's jets launched a 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 rebels, worn down by a succession of regime 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.

Violence has displaced 30,000 people in Idlib and the adjacent province of Hama this month alone.

The UN has warned that a large-scale military operation against the rebel-held province could create "the worst humanitarian catastrophe with the biggest loss of life in the 21st century".