Washington: Amid hurried diplomacy that signalled allied military strikes may be imminent, US President Donald Trump has said he will take swift action in retaliation to the alleged weekend chemical attack on civilians in Syria.

The Trump administration, backed by France and Britain, on Monday began making a circumstantial case that Syria and its Russian and Iranian partners bore direct responsibility for the weekend deaths of over 50 people in the opposition-held town of Douma, outside Damascus, the Washington Post reported.

"It was an atrocious attack. It was horrible," Trump said at the start of a Cabinet meeting where possible military action was discussed.

Options included the sort of largely symbolic airstrike Trump ordered a year ago in response to a similar chemical attack blamed on Syrian President Bashar al-Assad, or a wider and riskier assault, the New York Times said.

"We are studying that situation extremely closely. We are meeting our military... and we'll be making some major decisions over the next 24 to 48 hours," Trump said.

He said "nothing was off the table" in responding to the attack, suggesting that a new round of airstrikes could could be carried out as early as Tuesday. Any military response that would involve significant US ground forces remains unlikely.

The White House had earlier suggested that the US and France could act together, saying that during a phone call late on Sunday, Trump and French President Emmanuel Macron had agreed to "coordinate a strong, joint response".

"If it's Russia, if it's Syria, if it's Iran, if it's all of them together, we'll figure it out and we'll know the answers quite soon," Trump said.

At the UN, US Ambassador Nikki Haley and her French and British counterparts challenged the UN Security Council to resume an independent investigation of Syrian chemical weapons use.

"History will record this as the moment when the Security Council either discharged its duty or demonstrated its utter and complete failure to protect the people of Syria," she said. "Either way, the US will respond."

Haley accused Russia of helping Assad avoid international accountability. She called Assad a "monster" incapable of remorse and said his Russian enablers were incapable of shame.

Russia's UN representative Vassily Nebenzia, however, told the Security Council that images of the dead in Douma were staged and "fake news".