A doctor who treated Kremlin critic Alexei Navalny after he collapsed on a flight in Russia last year has gone missing, according to police.

Physician Alexander Murakhovsky left a hunting base in a forest in Omsk, Siberia, about 2,200km (1,370 miles) east of Moscow, in an all-terrain vehicle on Friday.

He has not been seen since.

Police in the region – which is vast but extremely sparsely populated – said emergency services, drones, a helicopter and volunteers on the ground were all involved in the search for him.

Mr Murakhovsky was the head doctor at the hospital in the Siberian city of Omsk that treated Mr Navalny, Russian President Vladimir Putin‘s most prominent critic.

He was subsequently promoted to the post of regional health minister.

Mr Navalny began feeling ill on a plane en route from Tomsk to Moscow, which made an emergency landing in Omsk on 20 August 2020 due to his serious condition.

After tense negotiations with the authorities, Mr Navalny was airlifted to Berlin in Germany, where he was treated.

Laboratory tests in three European countries, confirmed by the global chemical weapons watchdog, established that Mr Navalny had been poisoned with a Soviet-style novichok nerve agent.

Novichok: What is the nerve agent used to poison Putin critic Alexei Navalny?

The Kremlin has repeatedly rejected any suggestion that Russian authorities tried to kill him.

Mr Navalny was jailed in February on what he said were trumped up charges, after being detained upon his return to Russia from Germany, where he spent five months recovering from the poisoning.

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Last month, Mr Navalny said he would end a hunger strike after being warned that he was putting his life at risk, having refused to eat in prison for more than three weeks.

Thousands of protesters have been arrested in Russia since he was jailed.