"We are Kadyrovtsy," a group of bearded men in Donetsk proudly tell a CNN correspondent on camera. Kadyrovtsy are supporters of Chechnya's Moscow-backed leader, Ramzan Kadyrov. They are notorious for their brutality against opponents of the regime in the conflict-riddled region in southern Russia.

They said they traveled to Donbas, Ukraine's eastern industrial region, to fight against the Ukrainian army. Their call their battalion "Vostok" (East) - the same name as that of a fighting force that assisted in suppressing a separatist uprising in Chechnya several years ago.

Alleged Chechen casualties

The presence of such militants in eastern Ukraine has not been officially confirmed. The Ukrainian government has stated this to be true, but Russia has denied it. Kadyrov himself also denied the allegation, insisting there are no pro-Russian Chechen special forces in Donbas. However, he told Russian television that volunteer fighters from his region could be operating there.

An increasing number of people from the city of Donetsk have reported the presence of militants from the Caucasus in the Donbas region. Even the city's mayor, Oleksandr Lukyanchenko, has made mention of it. He said Chechens from the cities of Gudermes and Grozny were among those killed and injured in the clashes between pro-Russian separatists and Ukrainian troops near Donetsk's airport several days ago. Television footage showed corpses in coffins being loaded onto trucks for transport.

Russian defense policy expert Vladislav Shurygin said he did not believe the Vostok battalion consisted of special forces. "It is made up of volunteers," he said, adding that many of the fighters had no combat experience. "That's why so many of them died in the battle at the Donetsk airport."

According to Shurygin, only 10 percent to 15 percent of the separatists in eastern Ukraine have military training.

Not a united force

At this stage it is still hard to determine who is fighting on the separatists' side, and the number of insurgents in Donbas is also unclear. Some estimates put it at several thousand. Some of the fighters have made their way into the country crossing the porous border with Russia, which stretches for hundreds of kilometers.

The separatists do not have a homogenous command structure. The Vostok battalion, for example, is under the command of Alexander Khodakovsky, a former employee of the Ukrainian security agency. This is why some experts doubt that the battalion has anything to do with the Chechen squad of the same name. According to Oleg Orlov, head of Russian human rights organization Memorial, the group is only called Vostok because it is operating in eastern Ukraine. He pointed out that the original Vostok battalion was dissolved in 2008.

Orlov recently returned from a trip to Donbas. He said he did not see any Chechens among the pro-Russian fighters. "But there are men there from the Caucasus and Russia who make no secret of their nationality," he added.

Orlov said the people in eastern Ukraine would perceive North and South Ossetians as Chechens. "For most of them, bearded Caucasians are automatically Chechens," he explained.

Various origins

According to Mark Galeotti from the Center for Global Affairs at New York University, there are a number of different paramilitary groups operating in eastern Ukraine. Some of these, he said, include former members of the Ukrainian security agency who once worked for former President Viktor Yanukovych and joined the separatists following his ouster.

"Many of them belonged to the Berkut [Ukrainian special police]," said Galeotti. Yanukovych deployed Berkut units against the pro-European protest movement on Maidan Square, but the police force was later dissolved by the new government in Kyiv.

"Locals from eastern Ukrainians are also involved in the combat," said Galeotti. "They wear different camouflage uniforms." He added that these fighters obtained weapons by stealing them from police barracks - and that they pose a particular danger to civilians.

"They know how to handle a Kalashnikov [rifle], but do not know anything about military ethics: no alcohol and no violence against civilians," said Galeotti. According to him, many of the combatants are "bandits" with connections to organized crime.

Then there are also foreign volunteers and paid mercenaries, most of them from Russia. "Among them are well trained armed forces, but also adventurers and radical Russian nationalists fighting against Ukrainians out of conviction," said Galeotti, adding that many of them had modern Russian weapons and were probably financially supported by Russia.

Redeployment of Chechen fighters?

Galeotti said it was possible that members of the former Chechen Vostok battalion were among the mercenaries fighting in eastern Ukraine. The squad was formally under the control of the Russian Interior Ministry, but it was financed and managed by GRU, Russia's main military intelligence directorate.

"After the battalion was dissolved, GRU maintained contact with the men," said Galeotti. Some of them ended up joining the police force, some private security firms. Now, people like this can apply their skills to fighting in Donbas.

"Someone wanted to send forces to Ukraine that are more disciplined and effective than the separatists," said Galeotti. This is why experienced militants could form the backbone of the Vostok battalion.