Trump holds Syria, Russia 'responsible' for Douma massacre

US President Donald Trump holds Russia and Syria responsible for a suspected chemical attack that killed more than 40 people, the White House said on Wednesday, refusing to rule out direct military engagement with Russia.

White House Press Secretary Sarah Sanders dismissed Russian suggestions that Saturday's attack in Douma could have been faked and said Trump was still weighing military options in response.

"The intelligence provided certainly paints a different picture," she said of the Kremlin's theory. "The president holds Syria and Russia responsible for this chemical weapons attack"

Trump is believed to be considering standoff missile strikes against facilities related to the production and delivery of chlorine and sarin or sarin-like agents.

But many of Syria's most sensitive military facilities are protected by Russian missile defence systems or are located at bases where Russian, Iranian and Syrian personnel cohabit.

Sanders pointedly refused to rule out the possibility of direct military engagement with Russia - not shying away from the spectre of clashes between two nuclear superpowers.

"Once again, all options are on the table," she said.

Still, despite Trump's tweets early on Wednesday warning "gas-killing animal" Assad that "nice and new and 'smart!'" missiles were headed for Syria, Sanders said no "final decisions have not been made on that front".

Secretary of Defense Jim Mattis as well as military personnel responsible for America's sea-based Aegis missile defence system huddled at the White House on Wednesday to discuss options.

Trump spoke with French President Emmanuel Macron and UK Prime Minister Theresa May on Tuesday, whose government officials are also in the midst of extensive consultations with each other about launching a military strike as early as the end of this week.

The delayed US response is in stark contrast to last year's Khan Sheikhoun chemical attack, of which US intelligence agencies had video and other evidence of certain aspects of the actual attack, which involved the use of sarin gas. Trump responded by launching 59 precision guided Tomahawk Navy cruise missiles at a Syrian airfield within 72 hours.