Citing anonymous military and administration officials, The New York Times said Mattis had recommended Trump get approval from lawmakers before launching Friday's cruise missile barrage against three targets the Pentagon said were tied to Syrian President Bashar al-Assad's chemical weapons program.

"I have no idea where that story came from," Mattis told reporters as he greeted Qatar's defense minister, Khalid bin Mohammed al-Attiyah, at the Pentagon.

"I found nothing in it that I could recall from my own last week's activities."

Chemical weapons inspectors are waiting to go into Douma, near Damascus, to probe allegations of a chemical gas attack on April 7 that prompted last week's US-led response.

Mattis said the regime has previously used delays after such an attack to "try to clean up the evidence before the investigation team gets in. So it's unfortunate they were delayed."

Following the deadly Douma incident, Trump tweeted there would be a "big price to pay" after and promised missiles would be coming.

His remarks virtually ensured a 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.

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

Later Wednesday, White House spokeswoman Sarah Sanders denied the Times story and said Trump "appropriately ordered the strikes under his constitutional authorities."

In the days since the US-led strikes, which also saw British and French jets launch missiles, debate in Washington has continued about whether Trump has the legal authority to conduct strikes against the Syrian regime.

On Monday, a bipartisan group of senators introduced a bill that would update war powers that first were passed in the days after the September 11, 2001 attacks.

US forces have largely been operating under this so-called AUMF (Authorization for Use of Military Force) even though the Pentagon's mission has grown far beyond what was envisioned in the early days of hunting Al-Qaeda in Afghanistan.

Since 2001, presidents George W. Bush, Barack Obama and now Trump have relied on the authorities, along with a subsequent AUMF in 2002, as the basis for operations against armed Islamist groups.

One of the bill's authors, Republican Senator Todd Young, said it "recognizes the unique nature of the Islamist terrorist threat, while also recognizing that Congress must exercise robust oversight."

Former president Barack Obama faced sharp criticism when he tried and failed to have Congress approve a plan to attack Assad after the Syrian leader crossed Obama's "red line" and used chemical weapons in 2013.