US Secretary of State John Kerry has urged all sides in the Ukrainian conflict to prevent breaches of a ceasefire agreed earlier this year.

He was talking at a joint briefing with Russian Foreign Minister Sergei Lavrov in the Russian Black Sea resort of Sochi

"The resort to force by any party at this time would be extremely destructive", he said. 

Earlier, Mr Kerry met President Vladimir Putin to probe Russia's willingness to curb its involvement in Ukraine and its backing of Syrian president Bashar al-Assad.

Mr Kerry met Russian Foreign Minister Sergei Lavrov for more than four hours before he sat down with Mr Putin, in what was the highest-level US visit to Russia since the Ukrainian crisis began in the autumn of 2013.

While they were expected to discuss issues including the Iran nuclear talks, Yemen and Libya, the trip appeared designed as much to maintain contact as anything else given that US-Russian relations are at their lowest ebb since the Cold War.

"It's important for us to keep these lines of communication open. It's important to try to talk to the senior decision-maker," a senior US State Department official who briefed reporters as Kerry travelled to Russia said.

"We have a lot of business that we could do together if there is interest," said the official, who was speaking anonymously.

Mr Putin's spokesman, Dmitry Peskov, said the meeting was a positive step.

"Through dialogue, it is possible to find ways for a normalisation, closer coordination in dealing with international problems," Mr Peskov told reporters before the talks began.

However, he added: "Russia was never the initiator of this cooling of relations."

Kremlin adviser Yuri Ushakov told reporters that the meeting yielded no breakthrough, but added Mr Putin would like relations between the two nations to return to normal.

Mr Lavrov said his meeting with Mr Kerry helped each other understand each other better, also saying it is necessary to avid steps that could further harm relations.

Meanwhile, allies of murdered Russian opposition leader Boris Nemtsov have unveiled a report based on research he collected that they claimed proved Russian military involvement in Ukraine.

"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".

"All key military successes of the separatists were ensured by regular Russian army contingents," Mr Yashin said of the conflict between Ukrainian forces and separatists in the east of the country, which has resulted in over 6,100 deaths since last April.

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 65-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 conflict and came ahead of negotiations on the conflict.

The first deployment of the regular Russian army to eastern Ukraine resulted in the deaths of soldiers, with "more than 150 coffins" returning to Russia, while in the winter "at least 70 troops died," Mr Yashin said.

Russia also 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, he said.

Russian authorities have fiercely denied accusations they are backing the separatist rebellion in Ukraine.

The Kremlin refused to comment on the claims, with spokesman Dmitry Peskov telling news agencies: "I am not familiar with the report so I have nothing to say."