UK government officials are warning that more terrorist attacks in Russia are "very likely to occur" in the run-up to, or during, next week's Winter Olympics in Sochi, where 88 countries will be competing.

A threat assessment seen by the BBC names a Caucasus group, Imarat Kavkaz (IK), as posing the main danger to the games on the Black Sea coast, saying it has repeatedly expressed a desire to target them.

It points out that in July 2013 the group's fugitive leader, Emir Doku Umarov, called on his followers to do what they could to disrupt the games.

The document also points out that IK has no track record of attacking non-Russian interests - its fight is with Russia rather than with the West.

Meanwhile, US is taking security seriously, posting two of its warships offshore in the Black Sea, offering "unspecified air assets", deploying FBI agents and sharing counter-terrorism intelligence with Russia.

NBC reports that an internet message from a group claiming to be behind attack in Russia last year promised further violence against the country though the group did not specifically mention the Olympic Games.