Donald Trump will make "major decisions" on a US response to a "barbaric" and "heinous" chemical weapons attack in Syria over the next 24 to 48 hours.
The US President revealed he is "studying the situation extremely closely" as he contemplates a reaction after at least 70 people were reported to have been killed and 500 injured in the city of Douma on Saturday.
Speaking in the White House, Mr Trump said: "It was an atrocious attack, it was horrible, you don't see things like that – as bad as the news is around the world, you just don't see those images.
"We are studying the situation extremely closely, we are meeting with our military and everybody else and we'll be making some major decisions over the next 24 to 48 hours.
"We are very concerned when a thing like that can happen. This is about humanity, we're talking about humanity. And it can't be allowed to happen.
"So, we'll be looking at that barbaric act and studying what's going on."
The Syrian regime has denied being responsible for the attack, while Russia – which backs the country's President Bashar al Assad – also rejected allegations the Syrian government was to blame.
Mr Trump revealed the US is "trying to get people" into Syria as he asked: "If they're innocent, why aren't they allowing people to go in and prove? Because, as you know, they are claiming they didn't make the attack."
He added: "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.
"So, we're looking at that very, very strongly and very seriously."
Mr Trump spoke hours after Theresa May blasted Russia for enabling "long-standing international norms" on chemical weapons to be broken.
The Prime Minister said she found pictures of Syrian children foaming at the mouth "horrific" and also branded the raid "barbaric", suggesting President al Assad could be behind it.
Speaking at a news conference on a trip to Denmark, Mrs May declared that "incidents like this cannot be tolerated".
"The UK utterly condemns the use of chemical weapons in any circumstances and we must urgently establish what happened on Saturday," she said.
"If confirmed, this is yet another example of the Assad regime's brutality and brazen disregard for its own people and for its legal obligations not to use these weapons.
"If they are found to be responsible, the regime and its backers, including Russia, must be held to account."
Mrs May added that the attack "fits into a troubling wider pattern" of "abuse of long-standing international norms" on chemical weapons
Attacking Russia for its "repeated vetoes" at the UN, Mrs May said they had "enabled these rules to be broken… and removed mechanisms that allow us to investigate" chemical weapons attacks in Syria.
She declared: "This must stop."
On her trip to Sweden later on Monday, the Prime Minister was quizzed on Mr Trump's comments and asked whether the UK would join a possible international coalition in favour of renewed military action in Syria.
Mrs May replied: "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."
The Prime Minister repeated the same answer when asked if she would consider recalling Parliament from its current recess to debate military action in Syria.
Mrs May promised to work with other countries ahead of a showdown at the UN Security Council later on Monday.
US defence secretary Jim Mattis earlier raised the possibility of military action if Syria is found to be responsible.
Asked whether he was ruling out launching retaliatory airstrikes, he said: "I don't rule out anything right now."
After the Syrian regime used sarin nerve agent during an attack in Khan Sheikhoun in April last year, Mr Trump launched retaliatory missile strikes against a Syrian government airbase.
Foreign Secretary Boris Johnson spoke to acting US secretary of state John Sullivan and French foreign minister Jean-Yves Le Drian on Monday.
The Foreign Office said Mr Johnson and Mr Sullivan "agreed that, based on current media reports and reports from those on the ground, this attack bore hallmarks of previous chemical weapons attacks by the Assad regime".
A spokesperson added: "They reiterated their commitment to standing up for the Chemical Weapons Convention and to ensuring that those responsible for this horrific attack are held to account.
"They underlined the importance of the UK, the US, and France remaining in close touch."
Former NATO general secretary Anders Fogh Rasmussen told Sky News a failure by Western allies to act over the attack would "send a very bad signal, not only to the Assad regime but to all the bad guys around the world".
Calling for Mr Trump to launch strikes against the Syrian regime, Mr Rasmussen said it would be "extremely important" for the UK to join the US in any action and claimed "military pressure can facilitate political and diplomatic solutions" in the Syria's long conflict.
"The UK should appreciate widespread solidarity after the Salisbury attack," he said.
"Syria is a tragic example of the cost of inaction. It's easy to criticise politicians who take action. You should also realise that the cost of inaction is incredibly high."
The Kremlin said President Vladimir Putin has warned German Chancellor Angela Merkel against "provocations and speculations" in a phone call on Monday.
Russian foreign minister Sergei Lavrov has also said such allegations that Syria was involved in such an assault were false and a provocation.
A Russian military taskforce in Syria claimed to have visited the hospital where patients were said to have been treated and found no evidence to back up reports of an attack.
They quoted a doctor and an ambulance driver who both reportedly said they had not dealt with anyone showing symptoms of chemical poisoning.
But an aid group with links to anti-Assad forces called it "one of the worst chemical attacks in Syrian history".
Volunteer rescue service The White Helmets, which also has links to opposition forces, said many of the victims were women and children.
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The group published graphic images showing a number of dead children who appeared to have been frothing at the mouth.
The Syrian American Medical Society also described patients foaming at the mouth, saying victims suffered corneal burns and smelled of a "chlorine-like odour".