Ambassadors of the Group of Seven (G7) leading industrialized states say they are "deeply concerned" about the situation of film director Oleh Sentsov and "other Ukrainian prisoners and detainees" in Russia.

"Their release, as part of a broader bilateral exchange of detainees, would be an important humanitarian step forward," the countries' ambassadors to Kyiv said in a joint statement released via Twitter on June 21.

The G7 consists of Canada, France, Germany, Italy, Japan, Britain, and the United States.

Separately, the U.S. Embassy in Kyiv urged Russia to act as the eyes of the international community were on the country that is hosting the World Cup soccer tournament.

"With the world watching the World Cup, Russia should allow access to all Ukrainian prisoners, including film director Oleg Sentsov, who is in his 2nd month of a hunger strike in a Russian prison," the embassy tweeted.

Ukrainian Ombudswoman Lyudmyla Denisova, who traveled to Russia last week, has not been allowed to meet with Sentsov and other Ukrainians considered as political prisoners by Kyiv.

Sentsov’s lawyer, Dmitry Dinze, who visited the imprisoned man on June 21, told Russian media that he had lost 13 kilograms, was drinking 3 1/2 liters of water a day, and was currently in the Arctic prison's medical center, where he is receiving nutrition through an IV drip.

Dinze added that Sentsov was experiencing problems with his heart and kidneys and that he had briefly been hospitalized on the 26th day of his hunger strike, now in its 40th day.

Earlier, the Russian state-run RIA Novosti news agency quoted Anatoly Sak, an ombudsman in the Siberian region, as saying he had visited Sentsov recently and that the activist was in satisfactory condition and alert.

There was no way to independently confirm the comments.

His lawyer also said that Sentsov had turned down a request by the European Court of Human Rights (ECHR) to stop his hunger strike.

Ukrainian President Petro Poroshenko spoke with his Russian counterpart, Vladimir Putin, via telephone on June 21, calling on him to allow Denisova access to the Ukrainian "hostages," according to Ukraine’s presidential website.

The Kremlin said that, during the call, initiated by Kyiv, the sides touched on the topic of prisoner exchanges and visits by monitors from each country to the other's prisons.

Sentsov, a vocal opponent of Russia's 2014 annexation of Crimea, is currently on hunger strike while serving a 20-year sentence in far-northern Russia.

A native of Crimea, he is demanding the release of 64 Ukrainian citizens he considers political prisoners.

The 41-year-old was sentenced in 2015 for conspiracy to commit terror acts, charges he and human rights groups say were politically motivated.

Western governments and rights organizations have called for Sentsov to be released, and the Russian human rights group Memorial considers him a political prisoner.

On June 20, the secretary-general of the Council of Europe urged Russia to free him "on humanitarian grounds."

"If there is a need for a request for pardoning him, I would gladly do it on the basis of the European Convention of Human Rights," Thorbjorn Jagland told Russian Ombudswoman Tatyana Moskalkova, the Interfax news agency reported after their meeting in Moscow.

Jagland made the call two days after a dozen leading names in the Russian arts called for President Vladimir Putin to pardon Sentsov.

But Kremlin spokesman Dmitry Peskov told reporters that the Ukrainian film director would have to ask for the pardon himself before it could be considered.

With reporting by AFP