THE prospect of a US military strike on Syria within the next 48 hours has increased after Donald Trump denounced as “heinous” the suspected poison gas attack on the Damascus suburb of Douma.

The US President vowed “major decisions” would be made soon, insisting: “We cannot allow atrocities like that.”

He stressed there would be “a big price to pay”. Asked if military action was possible, Mr Trump replied: “Nothing is off the table.” Last year, the US launched a military strike against a Syrian airfield following a chemical attack attributed to the Assad regime.

As Theresa May described the suspected chemical attack as “truly barbaric” she left open the possibility that Britain might consider joining any US-led military action.

Asked during a visit to Sweden on Monday evening if this might be the case, the Prime Minister said: "What we are currently doing is working urgently with our allies to assess what has happened here…

“If it is clear it is the responsibility of the Assad regime, it's yet another example of the callous and brutal way in which he's been treating his people. And we are clear that those responsible should be held to account.

"We are working urgently with our allies to assess what has happened. But, we are also working with our allies on any action that is necessary."

Mrs May said the Assad regime’s backers, such as Russia, must also be held to account, noting how the nerve agent attack in Salisbury was "a terrible incident".

She added: "It is Russia that has the capability, the intent and the motive to conduct this attack."

The PM was expected to talk to Mr Trump last night after the United Nations Security Council met to discuss the suspected poison gas attack.

Any intention to involve the UK military will undoubtedly mean a Commons vote. But in 2013, David Cameron, the then PM, lost an historic vote for action in Syria following a chemical attack, which is widely seen to have emboldened the Assad regime.

Moscow, meanwhile, claimed no chemical attack had taken place and blamed Syrian rebels for a “false flag” alert to draw America into taking action against the Assad regime. It denied any involvement.

According to the Syrian-American Medical Society more than 500 people were hospitalised in Douma, showing symptoms "indicative of exposure to a chemical agent". These included: burning eyes; foaming at the mouth; a blue tinge to the skin; breathing difficulties and "the emission of chlorine-like odour".

It was estimated around 50 people died in the suspected attack on Saturday evening but medical groups warned the numbers could rise as rescue workers gained access to basements, where hundreds of families had sought refuge.

However, Sergei Lavrov, the Russian Foreign Minister, said there was nothing to suggest a chemical attack had taken place.

“Our military specialists have visited this place and they did not find any trace of chlorine or any other chemical substance used against civilians,” he explained.

The Hague-based Organisation for the Prohibition of Chemical Weapons expressed deep concern about the alleged Douma attack, saying it would launch an immediate investigation.

Earlier, Russia blamed Israel for escalating the crisis by attacking a Syrian airbase known as T4 near the city of Homs. Iran said at least three Iranians were among the 14 dead in the missile attack. Jerusalem refused to comment.

In Washington, a sombre-looking President told reporters a decision on US military action would be made “probably by the end of the day”.

He went on: “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.”

The President noted how Vladimir Putin could bear responsibility for the suspected chemical attack and warned: “If he does, it’s going to be very tough, very tough. Everybody’s going to pay a price. He will; everybody will.”

At home, Jeremy Corbyn condemned the suspected chemical attack in strong terms but refused to say whether President Bashir al-Assad or Moscow were responsible.

“I condemn it, absolutely, I condemn the use of chemical or biological weapons in any scenario, anywhere in the world,” declared the Labour leader.

But his Shadow Cabinet colleague did point the finger at Damascus. The Shadow Foreign Secretary said: “What has happened in Douma looks to be just the latest abhorrent attack in Syria using chemical weapons; a war crime for which the Assad regime has been found responsible in the past and which we utterly condemn.”