MOSCOW -- Russian opposition activists published a report Tuesday that they claim proves that Russia is deeply involved in the war in Ukraine, seeking to counter overwhelming state media reports casting the events as a local uprising against the Ukrainian government.

Prominent Kremlin critic Boris Nemtsov was working on the report, entitled "Putin.War," after Russian President Vladimir Putin, at the time of his murder in February.

Drawing on media accounts, testimonies from relatives and other representatives of dead soldiers and confidential sources, the 64-page report maintains that hundreds of Russian troops have died fighting in a war that has cost Russia hundreds of millions of dollars.

Opposition activist Ilya Yashin, who presented "Putin.War" in Moscow, said that by conservative estimates at least 220 Russian soldiers had died in two battles in eastern Ukraine within the past year.

Yashin said the real number of losses may have been much greater but that he would give figures only for deaths that could be confirmed.

The Russian Defence Ministry has denied that any of its soldiers have fought in Ukraine, saying that the Russians who have joined the armed separatists were volunteers.

The report claimed the soldiers were released from their duties in the army and listed as volunteers. The Defence Ministry promised to pay compensation if the soldiers were killed or wounded, but failed to live up to its commitments, the report said.

Ilya Yashin, one of the opposition activists presenting the report, said Russian government actions have harmed the country's standing in the world and economic well-being.

"The policies pursued by Putin, the isolationism into which he has forced our country, this war he has waged against our brother nation, all of it goes against the interests of Russia," he said.

Nemtsov was shot dead on Feb. 27 in central Moscow. Five suspects, including a Chechen police officer, were arrested in the killing, but investigators haven't named a suspected mastermind and the motive remains unclear.

Yashin said he believed it was possible that Nemtsov may have been murdered because of his investigation.

"That is one of the theories of what may have been behind his killing," he said.

Yashin said researchers on the report had been subject to intimidation, but that they would not be deterred.

"They killed Nemtsov, so we took over. If they don't let us work, others will take our place," he said.

According to Nemtsov's sources, at least 150 Russian soldiers died in August 2014. Their relatives received 2 million rubles (now worth about $40,000) in compensation and signed non-disclosure documents.

Around 70 Russian soldiers died in January and February in fighting around the city of Debaltseve, the report said, adding that their family members were unable to receive compensation and appealed to Nemtsov for help.

Many Russian citizens engaged in fighting in eastern Ukraine have freely admitted where they come from, but stopped short of confirming they are fighting under Moscow's orders.