German doctors who flew in on a medical plane to evacuate Russian opposition figure Alexei Navalny were on Friday allowed to see him in a Siberian hospital, Navalny's aide said, calling it "a positive development".

"The German doctors who came on this flight... finally got access to him just several minutes ago," Navalny's chief of staff Leonid Volkov told reporters at a press conference in Berlin. 

A German NGO, the Cinema For Peace foundation, chartered a medical plane late Thursday that flew from Germany to the Siberian city of Omsk to airlift Navalny back to Berlin's renowned Charite hospital after he suffered a suspected poisoning.

But Russian doctors treating Navalny said Friday he was too unstable to be moved.

They also said they had found no evidence to back up claims that the anti-corruption campaigner and fierce Kremlin critic had been poisoned.

Volkov said the access given to German medical workers was "a positive development" that allowed for cautious optimism Navalny may yet be allowed out of the country.

But the aide stressed that Navalny's family still had "no diagnosis, no analysis" and no access to reliable data about his condition, accusing the Russian authorities of trying to conduct a "cover-up operation".

Describing the situation at Omsk hospital, he said: "The medical staff, the people in white robes, were literally substituted by people in grey suits in their offices."

"Our demand stays the same: we have to bring him to some place, to Berlin to Charite, where independent analysis can be done of what is happening to him," Volkov said.

He asked European leaders to help push for Navalny's transfer.

"We ask everyone to push harder for this, because it's really necessary."