Russia has warned the US of "grave repercussions" if it strikes the Syrian regime over a suspected chemical weapons attack.
After Donald Trump revealed he will make "major decisions" over the next 24 to 48 hours on a response to a "barbaric" and "heinous" attack, the US President was alerted to a possible Russian reaction to military action.
At a meeting of the United Nations Security Council in New York, Russia's envoy Vassily Nebenzia insisted there was "no chemical weapons attack", despite reports 70 people were killed and 500 injured in the city of Douma on Saturday.
He added: "Through the relevant channels we already conveyed to the US 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."
The US has said it will call for a new UN Security Council vote on a fresh inquiry into the use of chemical weapons in Syria.
President Bashar al Assad's regime in Syria, which is backed by Russia, has denied being responsible for the attack.
But Mr Trump earlier declared the US will "know the answers quite soon" over the Douma attack, as he suggested those to blame could be Syria, Russia, Iran or "all of them together".
The US President also warned "nothing is off the table" when asked if he would consider military action.
Meanwhile, Theresa May has suggested President al Assad is behind the attack.
The Prime Minister said that if confirmed as a chemical weapons attack, it would be "yet another example of the Assad regime's brutality and brazen disregard for its own people".
Mr Nebenzia claimed Russia is being "unpardonably threatened" by the US and UK with a tone that "has gone beyond the threshold of what is acceptable, even during the Cold War".
He also claimed experts from the international chemical weapons watchdog should fly to Syria to probe the allegations, as he offered them protection in their work via Syrian authorities and Russian troops in the country.
However, Nikki Haley, the US ambassador to the UN, insisted America "will respond" whether the UN Security Council acts or not.
She said: "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."
As a permanent member of the UN Security Council, Russia can veto any substantive resolution.
Mrs Haley added "the monster" responsible for chemical weapons attacks in Syria "has no conscience", as she told Moscow its "hands are all covered in the blood of Syria children".
The UK's ambassador to the UN, Karen Pierce, told the meeting on Monday night that Britain believed the Syrian regime was responsible for the attacks.
She said: "It is truly horrific to think of victims, families sheltering underground when the chlorine found them."
Ms Pierce warned fellow nations on the UN Security Council they should "dread what we risk happening, that chemical weapons become a routine part of fighting".
In a dismissal of Russia's recent suggestion of a new Cold War between itself and the West, the UK envoy said: "This is not the Cold War, in the Cold War there was not this flagrant disregard for the prohibitions that are universal on the use of weapons of mass destruction."
"It is not us who wants to alienate Russia, she alienates herself," Ms Pierce added.
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As she called for an independent investigation into the suspected use of chemical weapons, Ms Pierce said: "We have nothing to hide, but it appears Russia and Syria and their supporters Iran do have something to fear."
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.