Russian troops regularly enter separatist-controlled territory in eastern Ukraine and serve there on a rotational basis, according to a volunteer who has been fighting with the insurgents since October.
Rustam, a Russian citizen from Tatarstan's Saba district who gave only his first name, made the claims in a telephone interview with RFE/RL's Tatar-Bashkir Service on July 20.
He said that regular Russian troops enter Ukraine's Luhansk and Donetsk regions for a period of one week before they leave and are replaced by fresh forces. Russia also regularly sends military instructors to Ukraine's east to train pro-Russian separatists, according to Rustam.
Moscow has officially denied any involvement in fighting in the Luhansk and Donetsk regions, the areas of conflict in which the United Nations estimates that more than 6,400 people have been killed.
Rustam, a 25-year-old Muslim, served a mandatory one-year military term in the Russian Army in 2009-10. Later, in 2011-14, he served on a contractual basis with a Russian military unit stationed in the North Caucasus region of Chechnya.
From there, after learning on the Internet how to link up with pro-Russian forces in eastern Ukraine, he joined the separatists as a volunteer in 2014.
Once on the ground, Rustam learned that every separatist unit in eastern Ukraine has one military instructor sent from Russia. He claims that these instructors were sent under the guise of contractors.
Overall, he estimates that every unit "has some 15 volunteers from Russia" fighting for it. The other fighters, he says, "are local people."
Asked why he wanted to join the separatists, Rustam echoed a line often used in Russian media.
“Well, it is about fascism here. Fascism. They have not fully eradicated followers of Hitler and Bandera here," he said. "I came here to fight against them."
Rustam said that his involvement was just, adding that many other Muslims -- including Azerbaijanis, Chechens, and others -- are among the separatist troops in eastern Ukraine.
He also says that he has never received any official payments for his service in Ukraine.
"I have never signed any documents related to payments. My name and surname has never been added officially to any form. There is no official information about me here as I came here just to help," Rustam said. "My commanders give me some money to buy clothes, food, and for some leisure. But officially, I have never been paid."
He added that the commanders give money in Russian rubles brought by armored cars on a daily basis. Rustam said that he does not know where the armored cars come from.
Written by Merkhat Sharipzhanov, based on an interview conducted by RFE/RL Tatar-Bashkir Service correspondent Nail Khisamiev