Vladimir Putin's options to retaliate if the West lets Ukraine use its long-range missiles to strike Russia could include striking British military assets near Russia or, in extremis, conducting a nuclear test to show intent, three analysts said, Paralel.Az reports citing Reuters.

As East-West tensions over Ukraine enter a new and dangerous phase, British Prime Minister Keir Starmer and U.S. President Joe Biden are holding talks in Washington on Friday on whether to allow Kyiv to use long-range U.S. ATACMS or British Storm Shadow missiles against targets in Russia.

Russian President Vladimir Putin, in his clearest warning yet, said on Thursday that the West would be directly fighting Russia if it went ahead with such a move, which he said would alter the nature of the conflict.

He promised an "appropriate" response but did not say what it would entail. In June, however, he spoke of the option of arming the West's enemies with Russian weapons to strike Western targets abroad, and of deploying conventional missiles within striking distance of the United States and its European allies.

Ulrich Kuehn, an arms expert at the Institute for Peace Research and Security Policy in Hamburg, said he did not rule out Putin choosing to send some kind of nuclear message - for example testing a nuclear weapon in an effort to cow the West.

"This would be a dramatic escalation of the conflict," he said in an interview. "Because the point is, what kind of arrows has Mr Putin then left to shoot if the West then still continues, apart from actual nuclear use?"

Russia has not conducted a nuclear weapons test since 1990, the year before the fall of the Soviet Union, and a nuclear explosion would signal the start of a more dangerous era, Kuehn said, cautioning that Putin may feel he is seen as weak in his responses to increasing NATO support for Ukraine.

"Nuclear testing would be new. I would not exclude that, and it would be in line with Russia shattering a number of international security arrangements that it has signed up to over the decades during the last couple of years," he said.

Gerhard Mangott, a security specialist at the University of Innsbruck in Austria, said in an interview he also thought it was possible, though in his view not likely, that Russia's response could include some form of nuclear signal.

"The Russians could conduct a nuclear test. They have made all the preparations needed. They could explode a tactical nuclear weapon somewhere in the east of the country just to demonstrate that (they) mean it when they say we will eventually resort to nuclear weapons."

Russia's U.N. ambassador Vassily Nebenzia told the U.N. Security Council on Friday that NATO would "be a direct party to hostilities against a nuclear power," if it allowed Ukraine to use longer range weapons against Russia.

"You shouldn’t forget about this and think about the consequences," he said.

Russia, the world's largest nuclear power, is also in the process of revising its nuclear doctrine - the circumstances in which Moscow would use nuclear weapons. Putin is being pressed by an influential foreign policy hawk to make it more flexible in order to open the door to conducting a limited nuclear strike on a NATO country.