The US has launched a scathing attack on Russia’s role in propping up the Syrian government of Bashar al-Assad with the country’s ambassador to the United Nations saying Washington “will respond” to a suspected chemical attack in Syria, regardless of whether the UN acts or not.

As Russia claimed first there was no evidence of a chemical attack in the Damascus suburb of Douma and then said the US had helped anti-Syria militants “stage” the incident which killed potentially dozens, the UN ambassador Nikki Haley, said the US was ready to act alone if necessary.

“We have reached the moment when the world must see justice done,” Ms Haley told an emergency session of the UN Security Council. “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.”

Her words came after president Donald Trump said that “nothing is off the table” when asked ahead of a cabinet meeting whether US military action was a possibility and that a decision would be swift.

He also took aim at Russia, Syria’s closest ally: “We cannot allow atrocities like that. Cannot allow it,” Mr Trump said. “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.”

Pressed on Russia’s role in the suspected chemical weapons attack, Mr Trump said Russian president Vladimir Putin “may” bear responsibility.

“He may. And if he does, it’s gonna be very tough. Very tough,” Mr Trump said of the US response. “Everybody’s gonna pay a price. He will and everybody will.”

Russian UN Ambassador Vassily Nebenzia told the Security Council that the US would face “grave repercussions” if it carries out an attack against Syrian government forces over the suspected chemical attack.

Almost precisely a year ago, Ms Haley appeared before the council and held up photographs of victims of a confirmed chemical weapons attack, allegedly carried out by the Syrian government, in the town of Khan Sheikhoun. This time, she said, she saw little value in producing photographic evidence as it would have no impact.

“Who does this? Only a monster does this. Only a monster targets civilians, and then ensures that there are no ambulances to transfer the wounded,” she said. “What’s the point of trying to shame such people …Pictures of dead children mean nothing to countries like Russia.”

Ms Haley spoke hours after a Syrian military airfield, known as T4, located close to the city of Homs, was struck in an attack that reportedly killed up to 14 people. The US and France both denied carrying out the strike. Many reports state the surgical strike was carried out by Israel, although the country has refused to comment.

Critics of the US and UK have accused them of double standards in the Middle East, having sold weapons to Saudi Arabia, who are involved in a bombing campaign in Yemen, while condemning Russia for its support of Mr Assad as part of a civil war that has been raging since 2011.

Mr Trump said the US would “probably by the end of today”, on how the US will respond to the alleged chemical attack. He said: “We are studying that 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.”

A year ago, in what was one of his first actions as Commander-in-Chief, Mr Trump responded to the deaths at Khan Sheikhoun by ordering the firing of 59 cruise missiles at Syrian targets.

Mr Trump was to meet with senior national security aides late in the evening to discuss options, with Monday being the first day on the job for his new national security adviser, John Bolton, who has previously advocated military action against Syria.

Donald Trump: 'nothing is off the table' for US response to alleged Syria chemical attack

For its part, Russia warned the US of “grave repercussions” if it carried out attack against Syrian government forces.

“There was no chemical weapons attack,” Mr Nebenzia said on Monday.

“Through the relevant channels we already conveyed to the US that armed forces 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.”

Mr Nebenzia also said investigators with the global chemical weapons watchdog, the Organisation for the Prohibition of Chemical Weapons (OPCW) should fly to Syria as early as Tuesday to look into accusations about the attack, offering them protection from Syrian authorities and Russian troops.

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“The Syrian authorities and Russian troops will provide conditions to travel to the areas of the alleged incident for them to familiarise themselves with the situation,” he said. Separately, the OPCW said it opened an investigation to determine what exactly had happened in Douma.

US Defence Secretary Jim Mattis, in separate remarks at the Pentagon, also suggested Moscow bore some blame. He criticised Russia for what he suggested was its failure to ensure the elimination of Syria’s chemical weapons arsenal under terms of a 2013 agreement.

Earlier, the British Prime Minister Theresa May said Russia must be “held to account” alongside Syria if Mr Assad’s government was found to be responsible for the attack.

At a press conference during a visit to Denmark, she said: “If they are found to be responsible, the regime and its backers, including Russia, must be held to account.”

Ms May said the attack in Douma was part of a “troubling wider pattern of acts of aggression” and abuse of international convention over chemical weapons and that Russia’s repeated use of its veto at the UN has enabled international rules on chemical weapons attacks to be broken and investigations hampered. “This must stop,” she said.

Mr Nebenzia had raised the issue of the poisoning of the Skripals in Salisbury at the UN to try and claim that those in the West were engaging in misinformation. Earlier, Ms May also spoke about the poisoning – which the UK has blamed on Moscow – saying that Russia had shown “similar recklessness last month with the use of chemical weapons”.

Asked if the UK would join any US-led military action in Syria, Ms May said: “What we are currently doing is working urgently with our allies to assess what has happened here.

“This has been an absolutely barbaric attack that has taken place,” she added.