Kiev protested to Moscow on Friday after a Ukrainian rights official was barred from visiting hunger-striking filmmaker Oleg Sentsov in a Russian prison amid growing concerns for his life. The 41-year-old director has refused food since May 14 to demand Moscow release all Ukrainian political prisoners, timing his high-profile protest to coincide with the month-long World Cup in Russia. As the football extravaganza thrust Moscow to the forefront of global attention, calls are multiplying to let Sentsov go, with US author Stephen King joining in. The Ukrainian foreign ministry accused Russia of "insidious steps" after Kiev's top rights official arrived at Sentsov's high-security prison in the town of Labytnangi on the Arctic Yamal peninsula, but was denied access to him.'

"Oleg is starving," Ukrainian foreign ministry spokeswoman Mariana Betsa said on Twitter. "We are protesting." Last week Russian President Vladimir Putin and Ukrainian leader Petro Poroshenko discussed a possible exchange of prisoners, raising hopes that Sentsov and others could be released during the World Cup. The two leaders then agreed that Kiev and Moscow's ombudsmen would visit their two countries' prisoners. Ukraine's ombudsman Lyudmyla Denisova said she went straight to Labytnangi because Sentsov was Kiev's top priority due to his deteriorating health. But Russian prison officials told her she could not see the filmmaker, she said. "According to some accounts, he is in critical condition," Denisova said in a statement, adding that talks with Moscow "have reached deadlock".