Crosses stand on the graves of unknown pro-Russian separatists at a cemetery in the eastern Ukrainian city of Donetsk, on February 16, 2015
Moscow (AFP) - Allies of murdered Russian opposition leader Boris Nemtsov on Tuesday unveiled a report he worked on claiming that hundreds of Russian soldiers have been killed fighting in Ukraine.
The report -- based on media articles and information from families of Russian soldiers allegedly killed in Ukraine -- repeats widespread allegations of Russian military involvement in Ukraine that Moscow has consistently denied.
"We gathered what we think is comprehensive proof of the presence of Russian troops," said Ilya Yashin, one of the authors of the report entitled "Putin. War" as he presented it to reporters in Moscow.
Nemtsov -- a former deputy prime minister who became a fierce Kremlin critic -- had started work on the report before he was gunned down just yards from the Kremlin in late February.
The 64-page report said the Russian army made two major incursions into Ukrainian territory in the summer of 2014 and in the winter of 2015. Both turned the tables in the fighting and came ahead of negotiations on the conflict.
"All key military successes of the separatists were ensured by regular Russian army contingents," Yashin said of the conflict between Kiev's forces and separatists in eastern Ukraine, which has resulted in over 6,200 deaths since April 2014.
The first major deployment of the regular Russian army to eastern Ukraine resulted in the deaths of soldiers around the strategic railway hub of Ilovaisk, with "more than 150 coffins" returning to Russia after fighting there.
- 'More than 150 coffins' -
After the winter incursion "at least 70 troops died," including around the town of Debaltseve, which was captured by the separatists shortly after the signing of the latest truce agreement.
Some of the information came from sources representing relatives of the soldiers who approached Nemtsov in February after the government did not deliver on its promise to compensate for the deaths, the report says.
The army forced the soldiers to officially end their contracts before deploying them across the frontier, the report says, and threatened relatives with criminal cases, forcing them to stay quiet.
Moscow further backed the recruitment of volunteers through army offices and oversaw the "transfer of military equipment" across the border, including the surface-to-air Buk system that was used to shoot down the Malaysia Airlines Flight MH-17 over the Donetsk region in July last year, Yashin said.
Nemtsov, who was serving as a regional MP at the time of his death, made several requests for information from the security services and border authorities regarding the flow of soldiers across the border to Ukraine, without receiving any response.
- Report 'irritated' Kremlin -
Russian authorities have fiercely denied accusations from Kiev and the West that they are backing the separatist rebellion in Ukraine.
The Kremlin refused to comment on the claims, with spokesman Dmitry Peskov telling Russian news agencies: "I am not familiar with the report so I have nothing to say."
Yashin said the final version of the report had a lot of debatable data removed so that "every letter, every word, every fact and every testimony" has been proven.
Nemtsov's colleagues believe the Kremlin is implicated in his brazen killing as all opposition figures are closely watched and often followed, while the bridge where he was shot is heavily monitored by police.
The Kremlin has denied any involvement and five men from Russia's volatile North Caucasus region have been arrested on charges of carrying out the hit.
Nemtsov's former colleague and friend, ex-prime minister Mikhail Kasyanov, said Nemtsov wanted to compile the report to offer an alternative source of information for Russians who are overwhelmingly dependent on state television.
"It is a document for Russian citizens to be able to compare information and understand what is happening," he said. "Propaganda... is not letting citizens make their own conclusions."
The investigation of Nemtsov's murder has stalled, Kasyanov said, as the mastermind is still at large.
The leading suspect currently in detention, former Chechen police officer Zaur Dadayev, has retracted his confession amid allegations he was tortured in custody.
Yashin said Nemtsov's work on the report could be one of the reasons for his killing and called for investigators to look into it.
"The work Nemtsov did was dangerous and provoked irritation in the Kremlin," he said.
The website of the report was opening slowly on Tuesday afternoon and Yashin said it was undergoing a cyber DDoS attack.
He and other authors of the report have received "threats" in the course of their work to finish the pamphlet, he said, calling it "psychological pressure".