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.

As East-West tensions over Ukraine enter a new and dangerous phase, British Prime Minister Keir Starmer and U.S. President Joe Biden held 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.

President 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.