Sept 13 (Reuters) - Russia's FSB security service said on Friday it had revoked the accreditation of six British diplomats in Moscow after accusing them of spying and sabotage work, signalling the Kremlin's anger at what it sees as London's vital role in helping Ukraine.

Britain described the accusations as "completely baseless", saying it was a tit-for-tat action after the UK expelled the Russian defence attache and removed diplomatic status from several Russian properties in May.

Russia announced the expulsions hours before talks in Washington between British Prime Minister Keir Starmer and U.S. President Joe Biden, a crucial step towards winning the go ahead for Kyiv to use long-range missiles against targets in Russia.

President Vladimir Putin said on Thursday that the West would be directly fighting with Russia if it allowed Ukraine to strike Russian territory with Western-made long-range missiles, a move he said would alter the nature and scope of the conflict.