Russia's ambassador to the UK has claimed Britain played a role in an attack on its warships - warning the country is "too deep" in the Ukraine war.
In an interview with Sky's Mark Austin, top diplomat Andrei Kelin claimed he had proof that UK special forces were involved in a Ukrainian drone assault on Russia's Black Sea fleet in Crimea and had handed 'evidence' to the British ambassador.
Asked to provide evidence of Russia's claims, Mr Kelin said: "We perfectly know about [the] participation of British specialists in [the] training, preparation and execution of violence against the Russian infrastructure and the Russian fleet in the Black Sea. We know that it has been done."
Pressed to give evidence to the public on Moscow's accusation the attack on the Russian fleet in the Black Sea was carried out under the guidance and leadership of British Navy specialists, Mr Kelin said it had been handed to the British ambassador and added that "it will become public pretty soon," perhaps today, perhaps tomorrow.
He added: "It is dangerous because it escalates the situation. It can bring us up to the line of I would say no return, return is always possible. But anyway, we should avoid escalation.
"And this is a warning actually that Britain is too deep in this conflict. It means the situation is becoming more and more dangerous."
Claims designed to distract from military failures, UK says
The UK government has said such claims are false and are designed to distract from Russia's military failures in Ukraine.
A spokesperson said: "In recent days, Russia has made a range of allegations against the UK, clearly designed to distract attention from Russia's illegal invasion of Ukraine; Russia's losses on the battlefield and its bombing of civilian populations and energy infrastructure without any regard for international law and the loss of innocent life.
"We do not plan to give a running commentary on these allegations; it is no secret that the United Kingdom has taken a public lead in our support to Ukraine - this has been enduring since Russia's illegal annexation of Crimea in 2014."
Moscow has cast Britain as a particularly insidious Western foil to Russia. President Vladimir Putin has said the UK is plotting to destroy Russia and carve up its vast natural resources.
Ambassador denies Moscow would use nuclear weapons
Speaking after Russia accused the West of "encouraging provocations with weapons of mass destruction", Mr Kelin denied Moscow would use nuclear weapons in Ukraine.
Mr Kelin said: "The nuclear war cannot be won and it should never be fought. And we stick strongly to this statement."
Asked if Moscow could use a tactical nuclear weapon in the conflict, Mr Kelin replied: "No. The world has every assurance that Russia is not going to use [a] tactical nuclear weapon in [the] Ukrainian conflict."
Moscow has been ramping up its nuclear rhetoric since it invaded Ukraine, most recently by accusing Kyiv of planning to use a "dirty bomb," though it did not offer evidence. Kyiv has denied it has any such plan.
The Russian Foreign Ministry said it feared the five declared nuclear powers were teetering "on the brink of a direct armed conflict".
It added: "We are strongly convinced that in the current complicated and turbulent situation, caused by irresponsible and impudent actions aimed at undermining our national security, the most immediate task is to avoid any military clash of nuclear powers."