US defense secretary says Syria appears to have heeded warning on chemical attack

Syria appears to have heeded a U.S. warning against staging any new chemical weapons attack as no such action has been launched, U.S. Defense Secretary Jim Mattis said on Wednesday.

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

Asked whether he believed Syrian President Bashar al-Assad's forces had called off any such strike completely, Mattis said: "I think you better ask Assad about that."

The White House issued the warning on Monday to Syria's leadership based on intelligence about what appeared to be active preparations at a Syrian airfield allegedly used for such an attack in April, U.S. officials said on Tuesday. Syria denied it carried out the attack.

Mattis did not directly speak to U.S. intelligence about Shayrat Airbase. However, he said that Syria's chemical weapons threat was larger than any single location.

"I think that Assad’s chemical program goes far beyond one airfield," he said.

Russia, Assad's main international backer, has denounced the warning and dismissed White House assertions that a strike was being prepared as "unacceptable," raising the tension between Washington and Moscow over the Syrian civil war.