Russia's Foreign Minister Sergey Lavrov has claimed there is a "real chance" of the war in Ukraine becoming World War 3.
Speaking to reporters on state television, he said NATO was "in essence" engaged in a proxy war with Russia as a result of supplying Kyiv with weaponry.
Moscow has been left outraged by the bolstering of Kyiv's supplies from outside sources, as Ukraine's military efforts have been boosted as a result.
Earlier this month, the Kremlin warned the White House that "unpredictable consequences" could come about if the US continued to send arms to Kyiv.
Mr Lavrov answered questions from reporters on whether the situation was comparable to the Cuban missile crisis in 1962, when tensions between the Soviet Union and the US came extremely close to a tipping point.
The Kremlin have been left outraged by the West's efforts to support Ukraine by supplying them with weaponry.
Reuters
Mr Lavrov said: "This is our key position on which we base everything. The risks now are considerable.
“I would not want to elevate those risks artificially. Many would like that. The danger is serious, real. And we must not underestimate it."
The threat of nuclear war has repeatedly been used by the Kremlin in an effort to deter the West from continuing their supply of arms to Ukraine.
Ukrainian President Volodymyr Zelenskyy has fostered a positive relationship with Boris Johnson.
Matt Dunham
Western leaders have pledged to delivering arms to Ukraine, but have ruled out of getting involved in the direct conflict, due to a risk of the conflict escalating.
Mr Lavrov added: "NATO, in essence, is engaged in a war with Russia through a proxy and is arming that proxy. War means war."
A senior military officer has said the UK is "fully equipped" to fight a war against Russia should NATO come under attack.
Air Marshal Richard Knighton, deputy chief of the defence staff, rejected claims made by one of his predecessors, General Sir Richard Barrons, that “NATO isn’t ready".
Intense Russian shelling has resulted in widespread damage across Ukraine, with no end to the conflict in sight.
Reuters
General Barrons told the House of Commons Defence Committee on April 19 that NATO “will have a call to make” should Russia attack members of the alliance, adding: “And that call would be easier if we had made any preparations at all to act in those circumstances at the speed required, and we have not.”
But AM Knighton told the same committee on Tuesday that NATO's mission was “to deter an attack” and that Russia “is learning a lesson about the cost of invasion” in Ukraine.
Asked directly by Conservative MP Richard Drax whether the UK had enough equipment and ammunition to fight Russia for more than a week, AM Knighton replied: “Yes".
He said: “General Barrons is a renowned commentator on such matters since he retired.
“The scenarios that we work through are many and varied, but if you are asking does NATO have the ability to blunt a Russian attack, yes it does."
However, he said he was unable to comment on claims by a retired US general that a joint exercise with the US Army in 2021 saw UK forces run out of ammunition in a week during a simulation of a Russian invasion.