Islamist militants in four regions of Russia's Caucasus have pledged allegiance to the Islamic State group, according to a recording which was welcomed by IS after being posted online.

The news sparked fears that the jihadist group's influence is growing among the region's younger generation of Islamists, and that they may try to prove themselves by staging brutal attacks on Russian soil.

The voice recording posted on YouTube on Sunday said militants in Russia's Dagestan, Chechnya, Ingushetia and Kabardino-Balkaria regions had all sworn fealty to IS leader Abu Bakr al-Baghdadi.

"We testify that all mujahedeen of the Caucasus... are united in this decision and there are no disagreements among us on this issue," the male voice says, listing the four Russian regions in the recording both in Arabic and Russian.

An IS spokesman on Tuesday welcomed the news, naming a young local warlord as the group's Caucasus leader.

"We congratulate the soldiers of Islamic State in the Caucasus... We congratulate them for making allegiance to the caliph," Abu Mohammed al-Adnani said in a recording of his own.

"He accepts your allegiance and names Sheikh Abu Mohammad al-Qadari as (governor) of the Caucasus."

Photos on jihadist websites identified al-Qadari as Rustam Aselderov, a former "amir" for the Dagestan region in the Caucasus Emirate insurgent group who was ousted after pledging allegiance to Baghdadi in December, becoming the first major leader to do so in the Caucasus.

He is a "young man who represents what the insurgency is today" who has no serious religious education and is oriented towards jihad, said Varvara Parkhomenko, a consultant for the International Crisis Group (ICG) and an expert on the North Caucasus.

Parkhomenko said the pledge is the latest event in the evolution of the Caucasus insurgency from a nationalist regional force to part of a global phenomenon where few members participated in the wars for Chechen independence over the last two decades.

"When the insurgency was local, they had to take into account that they act on their own territory, brutal attacks were not supported by the population," she said. "Now if they get tied up to foreign structures, such methods may become relevant."

"This is a very serious process and a challenge for Russia," she said.

- 'Attacks in Russia' imminent? -

Alexei Malashenko, an expert on religion and security at the Carnegie Center in Moscow, said the latest pledge could be mere words, but could also lead to attacks in Russia by IS converts, who are essentially adopting a foreign agenda.

"Since they've joined, now they have to somehow show themselves, with attacks in Russia," he said. "I don't exclude future attacks."

Though it is relatively far from its domain, the Islamic State group has shown interest in Russia in the past, even launching a Russian-language glossy magazine last month.

Last year, IS militants issued a threat to President Vladimir Putin, vowing to oust him and "liberate" North Caucasus.

On Wednesday, the deputy secretary of Russia's security council Yevgeny Lukyanov estimated there are "up to 2,000" Russians fighting with IS, adding that many "pretend to be tourists who lost their documents" when they return, usually via Turkey.

Early this month Turkish authorities detained a 19-year-old female student from Moscow who quit philosophy studies to fly to Istanbul and tried to cross into Syria.

Parkhomenko of the ICG said that the new generation of Caucasus Islamists may not be after the same goals as the old guard who fought to create a state for themselves.

"Now many are ideologically motivated," she said. "They want to die and go to heaven."