US says Syria warnings 'prevented chemical attack'

Syrian regime President Bashar al-Assad appears to have heeded US warnings not to carry out a chemical weapons attack, Defence Secretary Jim Mattis said on Wednesday.

Earlier this week, the White House had claimed Assad may be preparing another chemical attack, warning the Syrian regime that it would pay a "heavy price" if it went ahead with such an assault.

"It appears that they took the warning seriously," Mattis said. "They didn't do it," he told reporters flying with him to Brussels for a meeting of NATO defence ministers, Reuters reported.

US officials said they had recently seen activity at Shayrat airfield, which had been targeted by a US cruise missile strike on 6 April in retaliation for a deadly chemical weapons attack blamed on President Bashar al-Assad.

Asked if he believed the Syrian regime had completely cancelled plans for a chemical attack, Mattis replied: "I think you better ask Assad about that."

The US defence secretary added that Syria's chemical weapons threat extended far beyond a single location at the airfield.

The Assad regime had not commented on US warnings earlier this week, but state-run television said the allegations were fabricated, Reuters reported. 

A day after the US warnings, French President Emmanuel Macron said he had agreed to work together with President Donald Trump to formulate a common response in case of a new chemical attack in Syria.

"They discussed the Syria dossier and the necessity to work on a common response in case of a chemical attack," Macron's office said in a statement, without providing further details.

Russia, a key backer of Assad, said on Wednesday that it would respond if the US took pre-emptive measures against Syrian.

"We will react with dignity, in proportion to the real situation that may take place," he said at a news conference in the city of Krasnodar, Reuters reported.

Syria's war began in March 2011 with anti-government protests that spiralled into a complex and devastating conflict that has killed more than 320,000 people.

Russia is flying a bombing campaign in Syria in support of Assad, while the US is heading a coalition mainly targeting the Islamic State group.