Russian opposition leader Alexei Navalny, who the West believes was poisoned with a Soviet-era nerve agent, has been discharged from hospital after a month, his doctors in Berlin said Wednesday.

European leaders have demanded explanations from the Kremlin since Navalny, 44, fell ill after boarding a plane in Siberia in August and was hospitalised there before being flown to Berlin.

The Kremlin critic and anti-corruption campaigner spent 32 days in Charite hospital in the German capital, including 24 days in intensive care, before his release on Tuesday, the hospital said.

"Based on the patient's progress and current condition, the treating physicians believe that complete recovery is possible," Charite said in a statement, adding however that it remained too early to assess any long-term effects of his severe poisoning.

Russian doctors who first treated him said their tests did not find any toxic substances.

But Germany said toxicology tests provide "unequivocal proof" that he had been poisoned by the military-grade nerve agent Novichok, which was also used to poison ex-double agent Sergei Skripal in Salisbury, Britain in 2018. He survived.

France and Sweden have since said tests they ran independently corroborate Germany's conclusions.

European leaders have demanded explanations from Moscow, with Chancellor Angela Merkel saying that "only Russia can and must" provide answers on the poisoning.

The Kremlin has however rejected international calls for an investigation, calling any allegations that it was behind the poisoning "absurd" and "unsubstantiated".

'Cunning plan'

The poisoning of Navalny has heightened tensions between Russia and the West, in particular aggravating the relationship with Germany.

Merkel had always insisted on keeping channels of dialogue open with Moscow but she has sharpened her tone lately, with Navalny's case following a murder in a central Berlin park last year that German prosecutors say was ordered by Russia.

Navalny collapsed last month on a flight from Tomsk to Moscow after a campaign trip to support opposition candidates in local elections.

In his first blog post since emerging from coma, Navalny said on Monday that the three European labs had found Novichok "in and on my body".

He noted that Russia had still not opened an investigation but that he "did not expect anything else."

Amid his recovery, his spokeswoman Kira Yarmysh has said the opposition leader planned to return to Russia.

"No other option has ever been considered," she had told AFP 

Navalny aides said Thursday that German experts found Novichok nerve agent on a water bottle taken from the hotel room where he stayed before being taken ill. 

The bottle appears to have been key evidence for Germany's conclusion that the lawyer and outspoken critic of President Vladimir Putin was poisoned with the military-grade nerve agent.

Pressure has since been mounting on Putin, with France's President Emmanuel Macron urging him to shed light on the case.

On Tuesday, Le Monde newspaper reported that Putin had suggested that Navalny may have taken the poison himself "for a non-specified reason".

Reacting to the report, Navalny said in a sarcastic post on Instagram that "that's a good lead".

"I cooked Novichok in the kitchen. Swallowed some from my hip flask on the plane. Went into a coma... 

"My cunning plan was to die in a hospital in Omsk, where at the morgue the autopsy would have concluded 'cause of death: lived long enough'. But Putin saw right through me. 

"And as a result, I, like an idiot, was in a coma for 18 days, and didn't get what I wanted. The provocation failed," he wrote.