A file photo of Russian President Vladimir Putin shaking hands with Ukrainian President Petro Poroshenko, right. Picture: Sergei Bondarenko/Pool

Moscow, Russia - Russian leader Vladimir Putin and Ukrainian President Petro Poroshenko discussed Saturday a possible exchange of prisoners, the Kremlin said, raising hopes that hunger-striking film director Oleg Sentsov and others could be released ahead of the start of the World Cup next week.

"Special attention was paid to humanitarian issues including an exchange of people being held" by the two sides, the Kremlin said in a statement following the rare phone conversation by the two leaders.

Putin stressed that Russian journalists arrested in Ukraine should be released immediately, the Kremlin said, adding that the conflict in eastern Ukraine was also discussed.

Poroshenko for his part stressed that Sentsov, who has been on hunger strike since May 14, and other "Ukrainian political prisoners" should be released "as soon as possible."

"The head of the Ukrainian state drew attention to Ukrainians' hunger strikes and expressed concern over their deteriorating health," his office said in a statement.

Both Moscow and Kiev said the two leaders agreed that a rights ombudsmen would visit the two countries' prisoners "in the near future". No other details were provided.

Sentsov, who is serving a 20-year sentence on terrorism charges in Russia's far north, is demanding that Moscow release dozens of Ukrainian political prisoners languishing in Russian prisons.

His supporters said the father of two was prepared to die in prison, clouding Russia's chance to shine on the world stage during the World Cup which begins on Thursday.

Ukrainian activist Oleksandr Kolchenko, who is serving a 10-year sentence in a Russian prison after being convicted of terrorism together with Sentsov, also launched a hunger strike in solidarity with Sentsov.

He however halted his protest earlier this week due to his poor health.

Putin and Poroshenko spoke on the phone ahead of talks Monday in Berlin between the Russian and Ukrainian foreign ministers that will also involve Germany and France.

The talks are aimed at reviving a stalled peace process in eastern Ukraine. Efforts to put an end to a four-year conflict between Ukranian government troops and pro-Russian rebels that has claimed more than 10,000 lives since 2014 have come to a halt.