Citing anonymous military and administration officials, the New York Times said Mattis had pushed Trump to get a green light from Congress ahead of launching last Friday's cruise missile barrage against three targets the Pentagon said were tied to Syrian President Bashar al-Assad's chemical weapons program.

According to the Times, Mattis met with Trump in a series of meetings and pushed him to get congressional authorization, stressing the need to link military operations to public support.

But in a short statement, Pentagon spokeswoman Dana White's office said the article's claim about Mattis pushing Trump for congressional approval is "blatantly false."

A Pentagon official told AFP that there was no debate, and that "everyone" agreed Trump had the authority needed to launch the strikes.

Chemical weapons inspectors are waiting to go into Douma, near Damascus, to probe allegations of a chemical gas attack on April 7.

The following day, Trump tweeted there would be a "big price to pay" after a "mindless CHEMICAL attack" and later promised missiles would be coming.

His remarks virtually ensured a speedy response to the alleged chemical attack, even though many US lawmakers have expressed reservations over further military engagement in Syria unless Trump can articulate a long-term strategy for the country.