Image copyright Reuters Image caption Russia is conducting the biggest security operation in the history of the Olympic Games

Whitehall officials are warning that more terrorist attacks in Russia are "very likely to occur" before or during the Winter Olympics in Sochi.

Eighty-eight countries will compete in the Games which begin next week.

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

A twin bomb attack on the city of Volgograd in December killed 34 people.

It was claimed by a militant Islamist group, based in Dagestan.

The assessment points out that in July 2013 the IK's fugitive leader, Emir Doku Umarov, called on his followers to do what they can to disrupt the Games.

'Ring of steel'

But the document questions whether the group has the capability to target such a specific event within a narrow timeframe. The Winter Olympics are scheduled to run from 7-23 February.

The British assessment is that Sochi would be far harder for terrorists to attack than Volgograd due to Russian military operations, with Russia pouring in over 37,000 extra troops and police and imposing a "ring of steel" around the Olympic venue, largely closing it off from public access.

Dressed all in black, the security guards are already a familiar sight around the Olympic park.

The authorities have also been hunting a suspected would-be suicide bomber named as Ruzanna Ibragimova, the 22-year old widow of a militant, whom it is feared could already be in Sochi.

Image copyright AP Image caption Posters of Ruzanna Ibragimova have gone up in hotels and at the airport in Sochi

There are concerns too there could be up to three other female suicide bombers at large, known as "black widows" since they were reportedly married to slain insurgents.

The British document points out that IK has no track record of attacking non-Russian interests, its fight is with Russia rather than with the West. It said there was a possible threat, though less likely, from Russian jihadists returning from Syria.

Earlier this month five countries' National Olympic Committees received letters threatening terrorist attacks but officials have dismissed these as a hoax.

However the 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.

The Foreign and Commonwealth Office says large-scale public events present an attractive target for terrorists and travellers at the Sochi games should expect increased police presence and enhanced security measures in and around both the Olympic venues and the wider Sochi area.

Visitors should maintain a high level of personal security awareness at all times and in all places, it says.

The FCO advises against all travel to Chechnya, Ingushetia and Dagestan and the districts of Budyonnovsky, Levokumsky, Neftekumsky, Stepnovsky and Kursky in Stavropol Krai.

It also advises against all but essential travel to North Ossetia, Karachai-Cherkessia and Kabardino-Balkaria.