(CNN) President Bashar al-Assad is warning the West against attacking Syria as President Donald Trump's international allies make preparations to join the United States in any military action against the regime in response to last week's suspected chemical attack.

British Prime Minister Theresa May's senior ministers agreed on the need for action against Syria at a Cabinet meeting on Thursday, but Downing Street did not specify what measures the UK would take.

May spoke with Trump on Thursday night about the international response to Syria, and they "agreed that the Assad regime had established a pattern of dangerous behavior in relation to the use of chemical weapons," according to a statement a Downing Street spokesperson shared with CNN.

"They agreed it was vital that the use of chemical weapons did not go unchallenged, and on the need to deter the further use of chemical weapons by the Assad regime. They agreed to keep working closely together on the international response," the statement said.

French President Emmanuel Macron said earlier that any French strikes would target chemical facilities in the wake of Saturday's attack on the city of Douma.

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Assad said the threats by Western countries against Syria were "on the basis of lies they have fabricated," denying any responsibility for the suspected chemical attack.

"Any possible action will only cause more instability in the region and threaten international security and peace," Syrian state TV quoted Assad as saying at a meeting with a high-level Iranian delegation in Damascus.

On Wednesday, Trump appeared to suggest airstrikes were imminent, warning Russia, Syria's key ally, to "get ready" for military action. But after being criticized for apparently telegraphing US intentions, Trump on Thursday attempted to blur the timing of any potential airstrikes on Syria.

"Never said when an attack on Syria would take place. Could be very soon or not so soon at all!" he tweeted, without ruling out attacks.

The suspected use of chemical weapons has drawn the West back into Syria's seven-year conflict. There are concerns, however, that further international intervention risks a confrontation between Russia and the United States. Russia has provided Assad with strong military backing.

What might France and the UK do?

Macron said France had proof the Syrian regime has used chlorine as a weapon, and told French TV channel TF1 that he was in close discussions with Trump. "Yes, every day since the beginning of the week, our teams are working closely together and we will have decisions to make when the times comes, when we will judge it to be useful and effective," Macron said.

Macron said earlier this week that France was ready to attack the "chemical capabilities" of the Assad regime.

In London, it was unclear whether May would seek parliamentary support for any UK involvement. May is not required by law to consult the House of Commons, but by convention, members of Parliament expect to be consulted.

In 2013, Prime Minister David Cameron lost a vote on airstrikes on Syria, thwarting an effort then by President Barack Obama to attack the Assad regime.

On Wednesday, opposition Labour Party leader Jeremy Corbyn urged May to recall Parliament -- which is on recess -- to hold a vote before taking any action.

Any potential strike against Syria could be carried out by extensive US and UK military assets already in the region, including two US Navy destroyers armed with Tomahawk cruise missiles . British media reported that May has also ordered British submarines to move within missile range of Syria.

International warnings

Turkish President Recep Tayyip Erdogan said he spoke to Trump on Wednesday and that he planned to talk with Russian President Vladimir Putin. While he condemned Assad for killing "innocent kids in the Douma chemical attack," he also warned against using Syria as a place to flex military muscle.

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"The developments in Syria led to a regional and worldwide security crisis. We are extremely uncomfortable about the fact that some countries who have confidence on their military power are using Syria as their arm-wrestling field," he said.

The Organisation for the Prohibition of Chemical Weapons tweeted that a team of its investigators will start work Saturday in Douma. Earlier, Syria's ambassador to the United Nations, Bashar Jaafari, told reporters in New York that those investigators were to arrive Thursday and Friday.

Russia called for a UN Security Council meeting on Friday, with Ambassador Vassily Nebenzia saying he believed a military strike on Syria would violate the UN charter and international law.

"We hope that there will be no point of no return, that the US and the allies will refrain from military action against sovereign states," he said.

The World Health Organization estimated Wednesday about 500 people could have been affected by the suspected chemical attack in Douma, citing reports from its health partners on the ground.

In a statement, UN Secretary-General Antonio Guterres called on the Security Council to reach an agreement on the "continued use of chemical weapons" in Syria and cautioned the situation could quickly spiral out of control if it didn't.

"Let us not forget that, ultimately, our efforts must be about ending the terrible suffering of the Syrian people," Guterres said.

Macron and German Chancellor Angela Merkel have expressed concern about the "erosion" of the international ban on chemical weapons in discussions, according to the German government.