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In his first letter successfully smuggled out of jail, he denounces Russia's "cowardly" war in Ukraine – a war in which Moscow maintains it isn't a participant, The Guardian wrote.

Sentsov's cousin, Natalia Kaplan, received the smuggled letter last month. She said it was the first direct communication from him since he was jailed.

The prison authorities make it difficult to send and receive post – a standard tactic to put pressure on inmates, she said. "Oleg doesn't complain," she said, adding that "snow has already fallen" at his colony.

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"For three years I've been sitting in a Russian prison. For those three years a war has been conducted against my country," Sentsov writes. "The enemy is fighting like a coward, vilely, pretending he's got nothing to do with this. Nobody believes it. But that doesn't stop him."

Read also"Plastic bags on face, electric shocks": human rights activists tell of systemic torture of Ukrainians in RussiaAt least 10 Ukrainians are serving long jail sentences in Russia following dubious trials. Others are in captivity in separatist-run eastern Ukraine. "There are many of us held in Russia and even more in Donbass," Sentsov writes. "Some have been freed. Others hope and wait."

In his letter, Sentsov says he does not want preferential treatment. "I want to remain just a surname on a list," he writes. He adds that there is relatively little he can now do for his country – except "hold on." He urges Kyiv not to cave in to the Kremlin on his account, "or to pull us out at any cost."

He concludes: "We're not your weak point. If we're supposed to become nails in the coffin of a tyrant, I'd like to become one of those nails. Just know that this particular nail will not bend."

At his trial, Sentsov said he had been beaten up by his interrogators, who put a bag over his head and told him to confess. He refused. He told his court hearing in Rostov-on-Don: "I don't know what your beliefs can possibly be worth if you are not ready to suffer or die for them."

Read alsoKremlin dubs Crimean journalists, activists as 'terrorists'The film director has two children, aged 13 and 12. He has declined visits by his family after observing that other prisoners "fall into terrible deep depression" once their loved ones are gone, Kaplan said. She described him as "very direct," "goal-oriented" and with a "strong sense of justice."

Sentsov is incarcerated at a strict penal colony in the Siberian region of Yakutia, about 3,500 miles from Ukraine's capital, Kyiv.

Sentsov, a film-maker and pro-Ukrainian activist, was arrested in Crimea in May 2014 soon after Russia's president, Vladimir Putin, annexed the peninsula. He had been helping to deliver food to Ukrainian soldiers marooned at their bases following Russia's takeover.

In 2015, a Russian court convicted him and fellow activist Alexander Kolchenko after what his family say was like a Stalin-era show trial.

They were accused of being part of a terrorist conspiracy, setting fire to the offices of a political party in Crimea's regional capital, Simferopol, and trying to blow up a Lenin statue, charges his lawyers say were absurd and fictitious.