Russia and the United States vowed to improve strained ties on Tuesday after "frank" talks between Russian President Vladimir Putin and top US diplomat John Kerry, but there was no concrete breakthrough after eight hours of meetings.

Kerry's trip to the Black Sea resort of Sochi was the highest level US visit to Russia since the Ukraine crisis erupted in late 2013.

The Kremlin did not qualify talks of President Putin and Kerry as a breakthrough, but as first signs of understanding that the two great powers should get their relations back on track, Kremlin aide Yuri Ushakov said, according to Tass.

And while there were no tangible agreements, there was a marked change in the tone between the two countries that for months have traded barbs and accusations.

"Today's meetings allowed us to better understand each other," Russian Foreign Minister Sergei Lavrov told reporters after meeting Kerry for four hours to discuss Ukraine, Syria and Iran, among other issues.

A post on Kerry's Twitter account called the meeting with Putin "frank" and "productive" before he headed to Turkey for key talks with NATO foreign ministers on Wednesday.

Kerry said that NATO members were focused on getting Russia and Ukraine to adhere to the terms of a cease-fire agreement reached in February.

"There is no substitute for talking directly to key decision-makers, particularly during a period that is as complex and fast-moving as this is," Kerry said after separate talks with Putin which also lasted four hours.

"Our president proceeded in the conversation from our fundamental interest in seeing bilateral relations get back to normal," the Kremlin's Ushakov said. "As it was emphasized, it meets the interests of Russia, and we hope the interests of the US, and is extremely important from the point of view of ensuring international stability and security."

"Proceeding from this, we consider the talks very useful, necessary and positive," he added.

AFP - AP