At least 220 Russian soldiers have been killed in east Ukraine, opposition activists say, in a report offering what they call "ample evidence" to rebut president Vladimir Putin's denial his troops are fighting there.

The report, the last project of murdered Kremlin critic Boris Nemtsov, also said Russia had spent more than 53 billion roubles ($1.3 billion) in 10 months to fund the conflict and deprived people of 2.75 trillion roubles ($68 billion) lost to inflation.

Nemtsov was shot dead in central Moscow in February and members of his party, the liberal RPR-Parnas, and journalists helped finish the 65-page report based on open source information and interviews with soldiers' families.

"The war with Ukraine is an undeclared war, a vile cynical war which amounts to a crime against all of the Russian nation," Ilya Yashin, an aide to Nemtsov, said.

"Putin will go down in history as the president who made Russians and Ukrainians foes.

"The main conclusion of our report is that we have collected and systematised [information] which we think provides ample evidence of the presence of Russian soldiers in Ukraine," he told a news conference to unveil the report, called Putin. War.

A spokesman for the Kremlin declined to comment on the report, saying he was not familiar with it and could not say if Mr Putin would look into it.

Mr Putin says there are no serving troops fighting in Ukraine, only volunteers.

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The West accuses Russia of providing arms and troops to the separatists fighting government troops, as well as giving them training and intelligence.

It stepped up sanctions on Moscow over the conflict, which has killed more than 6,100 people.

The West first imposed sanctions on Russian businessmen and officials after Moscow annexed Ukraine's Crimean peninsula in March 2014, before unrest spread to the east of the country.

Ties between Washington and Moscow have deteriorated since then, with Moscow accusing Washington of orchestrating last year's overthrow of a Ukrainian president who was supported by Russia.

On Tuesday, US secretary of state John Kerry met with Mr Putin and Russian foreign minister Sergei Lavrov Sochi for highest-level talks since the fighting began.

Speaking to reporters afterwards, the two foreign ministers accentuated the positives despite the fact that relations have sunk to their lowest level since the Cold War.

"We have an understanding that we need to avoid steps which are able to inflict a long-term harm to bilateral relations between Russia and the United States," Mr Lavrov said.

"There is no substitute for talking directly to key decision makers, particularly during a period that is a complex and fast moving as this is," Mr Kerry said.

Compensation offered to some families, report says

Reuters has collected testimony from soldiers and their families about Russian servicemen fighting in Ukraine, which also calls into question the Kremlin's arguments.

The report, which Nemtsov started after families of Russian troops killed in east Ukraine asked for his help, said at least 150 Russian servicemen had been killed in fighting around the village of Ilovaisk and elsewhere in the region last summer.

Their relatives received 3 million roubles ($75,000) in compensation provided they did not speak publicly about the deaths, according to the report.

Another surge in violence killed at least 70 Russian soldiers in clashes around the town of Debaltseve earlier this year, the report said, adding that the relatives of these troops were left without compensation.

Authors said Russian soldiers were mostly forced to quit the army officially before heading to east Ukraine. They earned up to 90,000 roubles ($2,240) a month.

The authors also estimated that Russia had spent 53 billion roubles ($1.3 billion) over 10 months to support Russian "volunteers" and local rebels as well as provide military equipment.

At a time of economic crisis when growth is being squeezed by sanctions, they said inflation — which is running at over 16 per cent — had eaten up about 2 trillion roubles' ($50 billion) worth of people's salaries and 750 billion ($19 billion) in savings.

Reuters