U.S. President Donald Trump and his national security aides on Thursday discussed U.S. options on Syria, where he has threatened missile strikes in response to a suspected poison gas attack, as a Russian envoy voiced fears of wider conflict between Washington and Moscow.

Worries about a confrontation between Russia, Syria's big ally, and the West have been running high since Trump said on Wednesday that missiles "will be coming" in response to the attack in the Syrian town of Douma on April 7, and lambasted Moscow for standing by Syrian President Bashar al-Assad.

Trump tempered those remarks on Thursday and even as he consulted with allies such and Britain and France, who could join in any U.S.-led strikes on Syria, there were signs of efforts to prevent the crisis from spiraling out of control.

"Never said when an attack on Syria would take place. Could be very soon or not so soon at all!" Trump wrote on Twitter, raising the prospect that an attack might not be as imminent as he seemed to suggest the day before.

Trump met with his national security team on the situation in Syria later in the day and "no final decision has been made," the White House said in a statement. "We are continuing to assess intelligence and are engaged in conversations with our partners and allies," the statement said.

People walk on rubble of damaged buildings in the besieged town of Douma, in late March. The clash in Syria has left hundreds of thousands dead. (Bassam Khabieh/Reuters)

That did not necessarily signal, however, that Trump was cooling to the idea of military action, especially given the high stakes in Syria. U.S. officials noted that Washington was still assessing intelligence and co-ordinating allies.

Trump spoke to British Prime Minister Theresa May on Thursday, and the two agreed "on the need to deter the further use of chemical weapons by the Assad regime," May's office said.

'We have proof,' France says

He was also due to speak with French President Emmanuel Macron, who said France had proof the Syrian government carried out the attack near Damascus, which aid groups have said killed dozens of people, and will decide whether to strike back when all the necessary information has been gathered.

Attack kills dozens in Douma, but government denies involvement 1:11

"We have proof that last week ... chemical weapons were used, at least with chlorine, and that they were used by the regime of Bashar al-Assad," Macron said, without offering details of any evidence.

Two U.S. officials familiar with an investigation of samples from Douma and the symptoms of victims said initial indications that a mix of weaponized chlorine gas and sarin were used in the attack appeared to be correct. But U.S. intelligence agencies have not completed their assessment or reached a final conclusion, the officials said.

Syria and allies reject allegations

Russia, Syria and its other main backer, Iran, have said reports of the Douma attack were fabricated by rebels and rescue workers and have accused the United States of seeking to use it as a pretext to attack the Syrian government. Russia said it deployed military police in Douma on Thursday after the town was taken over by government forces.

"They are the guarantors of law and order in the town," RIA news agency quoted Russia's defence ministry as saying. There were signs of a global effort to head off a direct confrontation between Russia and the West. The Kremlin said a crisis communications link with the United States, created to avoid an accidental clash over Syria, was in use.

Vassily Nebenzia, Moscow's ambassador to the United Nations, said he "cannot exclude" war between the United States and Russia and urged Washington and its allies to refrain from military action against Syria. "The immediate priority is to avert the danger of war," he told reporters. "We hope there will be no point of no return," the envoy said.

Never said when an attack on Syria would take place. Could be very soon or not so soon at all! In any event, the United States, under my Administration, has done a great job of ridding the region of ISIS. Where is our “Thank you America?” —@realDonaldTrump

A team of experts from the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons, was traveling to Syria and will start its investigations on Saturday, the Netherlands-based agency said.

It was not clear whether Trump and U.S. allies would wait for the results of the investigation before deciding on a possible strike.

U.S. Defence Secretary Jim Mattis told Congress he believed there was a chemical attack in Syria, but added a short while later that the United States had not made any decision to launch military action. He also suggested he was examining ways to prevent any strikes from triggering a broader conflict.

An image taken from Flight Aware's live tracker shows planes moving around Syria, as speculation grows about potential strikes against President Bashar al-Assad's regime. (FlightAware)

"I don't want to talk about a specific attack that is not yet in the offing. ... This would be pre-decisional," Mattis told the House of Representatives Armed Services Committee.

Moscow is estimated to have dozens of aircraft at its Hmeymim air base in Syria including fighters and bombers, as well as 10 to 15 warships and support vessels in the Mediterranean.

The Syrian government and Russian forces in Syria possess truck-mounted surface-to-air missile and anti-aircraft artillery weapons systems.

Nathaniel Miller, U.S. navy fire controlman first class, hugs his wife goodbye before boarding the guided-missile destroyer USS Bulkeley, part of the Harry S. Truman Carrier Strike Group deployment to the Middle East from Naval Station Norfolk, Va., on Wednesday. (U.S. Navy/Mass Communication Specialist 2nd Class Lindsey E. Skelton via Reuters)

Britain's May won backing from her senior ministers to take unspecified action with the United States and France to deter further use of chemical weapons by Syria. May had recalled the ministers from their Easter holiday for the meeting to discuss Britain's response to what she has cast as a barbaric attack that cannot go unchallenged.

Russian ships had left the Tartus naval base in Syria, Interfax news agency quoted a Russian lawmaker as saying. Vladimir Shamanov, who chairs the defence committee of the lower house, said the vessels had departed the base for their safety, which was "normal practice" when there were threats of attack.

Any U.S. strike would probably involve the navy, given the risk to aircraft from Russian and Syrian air defences. A U.S. guided-missile destroyer, the USS Donald Cook, is in the Mediterranean.