Bolton warns Syria's Assad not to use chemical weapons after US withdrawal

US National Security Advisor John Bolton has warned Syria's president Bashar al-Assad not to use the impending US withdrawal from the country as a pretext to use chemical weapons against Syrians, saying there is "no change" to the US position that their use is a "red line."

President Donald Trump said in December that US troops had succeeded in their mission to defeat Islamic State militants and were no longer needed in Syria.

Trump has twice carried out airstrikes in Syria in response to apparent chemical attacks, with the intention of deterring Assad.

"We've tried twice through the use of military force to demonstrate to the Assad regime the use of chemical weapons is not acceptable," Bolton said while en route to Israel.

"And if they don't heed the lessons of those two strikes, the next one will be more telling."

Trump's announcement about the intended troop withdrawal was greeted by surprise and condemnation from many US lawmakers and allies, and prompted the resignation of Defence Secretary Jim Mattis and the US special envoy for the anti-IS coalition in protest.

Secretary of State Mike Pompeo is following Bolton to the Mideast this coming week for an eight-country tour of Arab allies to shore up support for the administration's partners in the region.

Bolton is set to meet with Israel's Prime Minister Binyamin Netanyahu and other officials on Sunday before travelling to Turkey.

Agencies contributed to this report.