LATEST: Syrian government invites the Organisation for the Prohibition of Chemical Weapons OPCW to send a team to investigate the scene of the attack.

Donald Trump has said he’ll make a decision on the US response to the “heinous attack” in Syria in the next 24 to 48 hours as Russia’s UN ambassador warned American military action could lead to “grave repercussions”.

Trump’s timeline emerged as Theresa May said the UK was “working with our allies on any action that is necessary”, indicating Britain was four-square behind the US.

On Monday, the US President condemned a suspected chemical weapons attack on a rebel-held town in Syria that killed at least 42 people.

His comments came ahead of an emergency meeting of the United Nations Security Council on Monday – called by the US, UK and France among others – in the aftermath harrowing pictures showing medics desperately battling to keep gassed children alive.

Speaking at a Cabinet meeting, Trump said he was talking to military leaders and would decide who was responsible for the attack, listing Russia, Syrian President Bashar al-Assad’s government or Iran as possible perpetrators. 

Dismissing claims of Kremlin’s involvement alongside its Syrian government ally, Russian ambassador to the UN Vassily Nebenzia told the UN that his country was being “unpardonably threatened” by the US and UK.

“The tone with which this is being done has gone beyond the threshold of what is acceptable, even during the Cold War,” he said.

“There was no chemical weapons attack… Through the relevant channels we already conveyed to the U.S. that armed force under mendacious pretext against Syria – where, at the request of the legitimate government of a country, Russian troops have been deployed – could lead to grave repercussions.”

In the latest development, the Syrian government has invited the Organisation for the Prohibition of Chemical Weapons OPCW to send a team to investigate the scene of the attack.

“Syria is keen on cooperating with the OPCW to uncover the truth behind the allegations that some western sides have been advertising to justify their aggressive intentions,” state news agency SANA said, quoting an official source in the Foreign Ministry.

Ramping up the rhetoric, and in a warning to the UN to act, US ambassador to the UN, Nikki Haley, told the summit: “The United States is determined to see the monster who dropped chemical weapons on the Syrian people held to account.

“You have heard what the president of the United States has said about this. Meetings are ongoing. Important decisions are being weighed, even as we speak.

“We are on the edge of a dangerous precipice. The great evil of chemical weapons use that once unified the world in opposition is on the verge of becoming the new normal.

“The international community must not let this happen. We have reached the moment when the world must see justice done.

“History will record this as the moment when the Security Council either discharged its duty, or demonstrated its utter and complete failure to protect the people of Syria.

“Either way, the United States will respond.”

He called for inspectors from the Organisation for the Prohibition of Chemical Weapons to fly to Syria on Tuesday to visit the site of the alleged attack.

Trump said on Sunday after initial reports of an attack that there would be a “big price to pay”.

US Defense Secretary Jim Mattis said he would not rule out military action such as air strikes if blame was proven.

“I don’t rule out anything right now,” he told reporters in Washington.

In a day of fast-moving international diplomacy, British Foreign Secretary Boris Johnson spoke to his American counterpart, acting US Secretary of State John Sullivan, about the situation, with both agreeing that based on reports from the media and on the ground the attack “bore hallmarks of previous chemical weapons attacks by the Assad regime”.

“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,” a Foreign Office spokesman said.

Earlier, May hinted the UK would be prepared to take part in military action against the Assad regime, with the prime minister saying “initial reports” suggested the attack was another example of the “brutality” of the Syrian regime.

Asked if she was considering direct British military action in response, May told a press conference in Copenhagen the UK and its allies were discussing “what action is necessary”.

Later, during a visit to Sweden, May seemed to harden her line based on Trump’s 48-hours warning.

She said: “We, as I say, are working urgently with our allies to asses what has happened. But, we are also working with our allies on any action that is necessary.”

Trump condemned the “heinous attack”, saying: “It was an atrocious attack. It was horrible.”

He said he will be huddling with military advisers to consider US options and “nothing’s off the table”.

Trump said the US is still investigating the possible involvement of the Iranian and Russian governments in the strike.

“If it’s Russia, if it’s Syria, if it’s Iran, if it’s all of them together, we’ll figure it out,” he said.

He added of Russian President Vladimir Putin that “everybody’s going to pay a price — he will, everybody will”. 

Syrian opposition activists and rescuers said poison gas was used on the rebel-held town near the capital – an allegation strongly denied by the Assad government.

Families were reportedly found suffocated in their homes and shelters, with foam on their mouths.

Reports suggested more than 500 people, mostly women and children, were brought to medical centres with difficulty breathing, foaming at the mouth, and burning sensations in the eyes.

Johnson said in February that Britain should consider joining military action against Assad’s regime if there is fresh “incontrovertible” evidence he has used chemical weapons against his own people.