UPDATE 1-Russia responsible for Navalny's death, UN rights expert says

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UN holds diplomatic meeting on political prisoners

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UN expert worried more Russian opponents will die like Navalny

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Calls on countries to increase pressure on Moscow

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By Emma Farge

GENEVA, March 11 (Reuters) - The U.N. human rights expert on Russia said on Monday that the death of opposition politician Alexei Navalny was Moscow's responsibility as he was either killed in prison or died from detention conditions that amounted to torture.

Russian authorities say Navalny, President Vladimir Putin's most prominent critic, died on Feb. 16 in an Arctic prison of natural causes. Navalny's wife Yulia Navalnaya has accused Putin of having him killed, an accusation the Kremlin rejects.

"So the Russian government is responsible, one way or another, for his death," Mariana Katzarova told Reuters on the sidelines of an event on Russian political prisoners at the United Nations in Geneva. She cited long periods of solitary confinement which she said amounted to about 300 days, which could have caused "a slow death over several years".

Katzarova, who was appointed last year and has not yet been granted access to the country, also said other detainees in Russia could suffer the same fate as Navalny. She was "very worried" about opposition politician Vladimir Kara-Murza.

"Ever since the death of Alexei Navalny, there is no day passing without asking myself, who is the next Navalny?" she said. "And there will be a next Navalny, for sure, with this level of repression."

Katzarova, a Bulgarian former investigator for Amnesty International, is one of dozens of independent human rights experts mandated by the United Nations to report on specific themes or crises, though the only one reporting on one of the five states with a permanent seat on the U.N. Security Council.

Civil society groups say that around 600-1,000 political prisoners are being detained in Russia for voicing opposition to the Russian invasion of Ukraine or refusing to fight in it. Moscow rejects criticism of its domestic rights record.

In her address at the U.N. meeting earlier, where Nobel Prize-winning Russian journalist Dmitry Muratov also spoke, Katzarova called for more pressure on Moscow to release political detainees and investigate Navalny's death.

"We cannot afford to just be insulted by the human rights situation in Russia," she told the packed room of diplomats. "It's up to you to take steps, real steps for the protection of these political prisoners." (Reporting by Emma Farge and Cecile Mantovani; editing by Mark Heinrich)