Trump was to confer with French President Emmanuel Macron and British Prime Minister Theresa May later in the day on the action the allies might take, White House press secretary Sarah Sanders said.

"President Trump just finished a meeting with his National Security team to discuss the situation in Syria. No final decision has been made," Sanders said.

"We are continuing to assess intelligence and are engaged in conversations with our partners and allies. The President will speak with President Macron and Prime Minister May this evening."

AAP

The White House announcement came after May held an emergency cabinet meeting on the situation in Syria, where dozens of people were killed last weekend in a suspected toxic gas attack on Douma, the largest town in a former rebel stronghold outside Damascus.

Downing Street said May's top ministers had agreed on the "need to take action," despite polls showing the British public remains wary of military intervention.

Moscow has warned against any escalation that could trigger a conflict between the United States and Russia, and has called for an emergency UN Security Council meeting on Friday.

In Washington, Trump said earlier Thursday that a decision would come "fairly soon," though he equivocated in his tweets on the timing of any military strikes.

A French frigate, British Royal Navy submarines laden with cruise missiles and the USS Donald Cook, an American destroyer equipped with Tomahawk land attack missiles, have all moved into range of Syria's coast.

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US officials have refused to rule out direct military engagement with Russia, with the White House saying "all options are on the table."

But a special hotline for the US and Russian militaries to communicate about operations in Syria is active and being used by both sides, Moscow said Thursday.

UK agrees on "action"

The British government on Thursday agreed on the "need to take action" over a chemical weapons attack in Syria, Prime Minister Theresa May's office said.

"Cabinet agreed on the need to take action to alleviate humanitarian distress and to deter the further use of chemical weapons by the Assad regime," Downing Street said after May met her top team.

Ministers "agreed the prime minister should continue to work with allies in the United States and France to coordinate an international response," May's office added.

In Paris, France's Emmanuel Macron upped the pressure on Moscow by stating he had "proof" that the Assad's regime had used chemical weapons, and vowing a response "at a time of our choosing."

But across Western capitals opposition to military action also grew. US lawmakers questioned whether Trump has the legal authority to order strikes without Congressional approval and opposition parties voiced concern.

National security experts worried about whether strikes would actually serve to deter Assad.

Russia calls UN meeting

Russia has requested that the UN Security Council meet on Friday to discuss the threat of US-led military action in Syria, diplomats said.

The request came after Russian Ambassador Vassily Nebenzia said the priority was to avert US-led strikes on Syria that could lead to a dangerous confrontation between Washington and Moscow.

"The immediate priority is to avert the danger of war," said Ambassador Vassily Nebenzia following closed-door Security Council talks.

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Asked if he was referring to war between the US and Russia, he said: "We cannot exclude any possibilities unfortunately because we saw the messages that were coming from Washington."

"They know we are there, I hope, I wish there was dialogue through appropriate channels on this to avert any dangerous developments."

Trump has slammed Russia for its partnership with "Gas Killing Animal" Assad, spurring concerns that a US strike could lead to a conflagration with Russia, which has major military facilities at Tartus and Khmeimim and works cheek-by-jowl with Syriaforces that could be targeted.

US officials have refused to rule out direct military engagement with Russia, with the White House saying "all options are on the table."

But a special hotline for the US and Russian militaries to communicate about operations in Syria is active and being used by both sides, Moscow said Thursday.

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At the United Nations meanwhile, diplomats were mulling a draft resolution put forward by Sweden and obtained by AFP, that would dispatch a "high-level disarmament mission" to rid the country of chemical weapons "once and for all."

That may prove too little, too late.

The UN Security Council, tasked with maintaining international peace and security, has been riven, with Moscow virulently denying the Douma attack took place, or postulating that it was carried out by rebels.

The council has already failed to agree on a response to the attack in three votes and has been deadlocked throughout the Syrian civil war.