Islamist rebels from the Russian Caucasus who are fighting alongside Al-Qaeda-linked jihadists in Syria could pose a serious security threat to the 2014 Sochi Winter Olympics, analysts say.
Russia has pulled out all the stops to host the Olympic Games, its first major international sports event.
But experts fear that the return of war-hardened rebels from Syria may make it difficult to secure Sochi, which is close to the restive Caucasus region, during the high-profile event.
"Fighters from Russia are acquiring urban warfare experience in Syria. If they come back to Russia and get organised, it will be extremely dangerous," said Grigory Shvedov, the editor-in-chief of www.kavkaz-uzel.ru web magazine.
Despite the security measures which the Russian government is taking to protect the Olympic venue, Shvedov told AFP that "Sochi is quite vulnerable... to an attack by groups trained in urban battles in Syria."
In July, fears of an attack intensified when Islamist warlord Doku Umarov called on militants to stage attacks against the February Games.
In a video, he said jihadists must "exert maximum efforts" to prevent the high-profile international event from going ahead in Sochi, a city nestled between the Black Sea and the Caucasus mountains.
"If fighters from Syria come back (to Russia) and take up Umarov's call, this is a serious cause for concern," Shvedov said.
Umarov -- the Kremlin's number one enemy -- has claimed numerous deadly attacks in Russia in recent years, including those that claimed dozens of lives at Moscow's Domodedovo airport in 2011 and in the capital's subway in 2010.
"With the Olympics approaching, it is these people, crazed fanatics who are a real danger," said Alexei Malashenko, an analyst with Moscow's Carnegie Centre.
Russian President Vladimir Putin has also expressed fears about the return of Russian rebels who are currently battling the regime of Moscow ally Bashar al-Assad.
"We can only worry about the fact that hundreds of rebels from Western countries and even from Russia ... are fighting in Syria," the Russian strongman said in an article published last week by the New York Times.
"Who can guarantee that these bandits, with their experience, will not return in our country? It is a real threat for us."
Videos posted on YouTube (http://www.youtube.com/watch?v=2KMO_8EOD9g&feature=youtu.be) have so far shown only small groups of up to 30 fighters speaking Russian with Caucasian accents alongside the Syrian opposition rebels.
However, Malashenko said that estimates from various sources put the number of rebels from Russia fighting in Syria at between 300 and 2,000.
"In my opinion, there must be 1,000," he said, adding that this figure includes Islamists from the Caucasus republics of Chechnya and Dagestan and from Tatarstan in Central Russia.
The $50 billion Sochi Olympics project has been mired in scandals of corruption, overspending, and rights violations.
In July Russia's Interior Minister Vladimir Kolokoltsev vowed to set up in Sochi a "multi-tiered" security system in order to comply with International Olympic Committee requirements.
"We are ready to host any sort of sports event," Kolokoltsev said.
Russian authorities are also designating a vast mountain area above Sochi close to North Caucasus republics as a "restricted zone" and closing the border with Georgia's rebel region of Abkhazia during the Games.
Security experts have repeatedly warned that Sochi's location close to the restive Caucasus region and Georgia's breakaway region of Abkhazia means securing the venues without scaring tourists away could prove difficult.