United States President Donald Trump's national security adviser said on Monday the United States, Britain and France had agreed that another use of chemical weapons by the Syrian government would result in a "much stronger response" compared to previous air strikes.

"We've tried to convey the message in recent days that if there's a third use of chemical weapons, the response will be much stronger," National Security Adviser John Bolton said while fielding questions after a policy speech.

"I can say we've been in consultation with the British and the French, who joined us in the second strike, and they also agree that another use of chemical weapons will result in a much stronger response," Bolton said.

U.S. officials have said in recent days they have evidence that Syrian government forces are preparing chemical weapons ahead of a planned assault on Idlib, in the northwestern part of the country.

Marine General Joseph Dunford, the chairman of the Joint Chiefs of Staff, told reporters late last week he was involved in a routine dialogue with the White House about military options should Syria ignore U.S. warnings against using chemical arms.

Syria's Bashar Assad has massed his army and allied forces on the front lines in the northwest, and Russian planes have joined his bombardment of rebels there, in a prelude to a widely expected assault despite objections from Turkey.