At least 220 Russian troops were killed in two battles in eastern Ukraine in the past year, and the Russian authorities spent over $1.04 billion (1.25 billion euros) to aid the anti-government rebels in the eastern European country, Russian opposition activists from the group Open Russia said Tuesday.

According to the report, the victims' relatives received $40,000 (35,000 euros) as compensation from the Russian authorities, who also made them sign a non-disclosure agreement.

"We gathered what we think is comprehensive proof of the presence of Russian troops," Ilya Yashin, one of the authors of the research report entitled "Putin. War" in Moscow on Tuesday.

The document, based on research work by former deputy prime minister and one of Kremlin's fiercest critics, Boris Nemtsov, also said that "all key military successes of the separatists were ensured by regular Russian army contingents."

A conflict between Kyiv's forces and separatists in eastern Ukraine has resulted in over 6,100 deaths since last April.

The report was Nemtsov's last project before his murder. He made use of interviews and open source information to present a counter-narrative to Moscow's official position on the Ukraine war.

Since the initial stages of the armed conflict, Russia has maintained it has no troops on the Ukrainian soil, claiming that its support for the rebels is merely political.

Report details alleged Russian involvement

The report further said that Russian troops had made two major incursions into Ukrainian territory in the summer of 2014 and in the winter of 2015. Detailing the claims, Yashin told reporters in Moscow that the first deployment of the regular soldiers to eastern Ukraine resulted in deaths, with "more than 150 coffins" returning to Russia, while in the winter "at least 70 troops" died.

The report goes on to say that the Russian commanders further backed the recruitment of volunteers through their offices and oversaw the "transfer of military equipment" across the Russia-Ukraine border, including a surface-to-air Buk system. In July of last year, rebels used a surface-to-air system of the same make when they shot down Malaysia Airlines Flight MH17 over the Donetsk region.

Moscow insists that those who joined Ukraine's separatists were volunteers from Russia. The report, however, denies this claim and says that the Russian army first released its soldiers from their duties and listed them as volunteers.

In response to the report, Kremlin spokesman Dmitry Peskov told Russian media: "I am not familiar with the report so I have nothing to say."

Nemtsov's murder

Nemtsov was shot dead on February 27 in Moscow. Although the authorities have arrested five suspects, including a Chechen police officer, they haven't revealed the name of a suspected mastermind as the motive of Nemtsov's murder remains unclear.

The former prime minister's aides accuse President Vladimir Putin's government assassination. The authorities deny any involvement.

shs/kms (AFP, AP, Reuters)