The United States and allies France and Britain are currently finalizing plans for an expected strike against the regime of Syria President Bashar al-Assad after the alleged chemical attack that left more than 40 people dead.

"Some things are simply inexcusable, beyond the pale and in the worst interest of not just the chemical weapons convention but of civilization itself," Mattis told the House Armed Services Committee.

Lawmakers quizzed Mattis on what he made of President Donald Trump's confused messaging over Syria.

Trump last week said he wanted to pull out of the war-torn nation, but this week taunted Assad ally Russia with boasts of an impending missile strike.

The recognition of Syria's complexities "means you are at times ... going to see contrary impulses," Mattis said.

World powers have threatened a strong response to Saturday's attack, with French President Emmanuel Macron saying Thursday he had "proof" Syria's government was behind it.

Syria and Russia have both denied the accusations, and the global chemical watchdog said it would deploy a team to investigate.