Moscow, Aug 21 (IANS): Russian doctors said on Friday that Alexei Navalny, a leading opposition figure and a staunch critic of President Vladimir Putin, remains too ill to be transferred to Germany for treatment after a suspected poisoning.

Navalny has been in a coma since Thursday after he fell ill during a flight from Tomsk to Moscow and his plane made an emergency landing in Omsk, Siberia, the BBC reported.

After he was admitted to the Omsk Emergency Hospital, the 44-year-old's spokeswoman Kira Yarmysh tweeted that "Alexei has toxic poisoning".

But Anatoly Kalinichenko, deputy chief physician at the hospital, told reporters that no poisons were found Navalny's system, thus doctors do not think he was poisoned, TASS News Agency reported.

"No poisons or traces of poison have been found in his system. I suppose the diagnosis 'poisoning' is still at the back of our minds.

"But we do not think that the patient has been poisoned," Kalinichenko said.

Meanwhile, Alexander Murakhovsky, head doctor at the hospital, said Navalny's condition had improved a little, but that he was still unstable.

He added that legal questions would need to be resolved before Navalny could be moved, the BBC reported.

But Navalny's team said it was "deadly" for him to remain in the hospital.

"The ban on the transportation of Navalny is an attempt on his life, which is being made right now by doctors and the deceitful authorities who sanctioned it," Yarmysh tweeted on Friday.

The Berlin-based Cinema for Peace Foundation said it had organised a plane to pick up Mr Navalny and bring him back to Berlin, where the Charite hospital was ready to treat him.

It said the aircraft had medical equipment and a team specialised in treating coma patients on board.

The air ambulance arrived in the Siberian city of Omsk on Friday morning, according to flight tracking data.

Both Germany and France have said they were happy to help with treatment.

German Chancellor Angela Merkel said "he can receive from us all the help and medical support needed".

Polish Prime Minister Mateusz Morawiecki said he was "very concerned" by the reports of the alleged poisoning and the refusal to move Navalny.

A spokesperson for Putin said on Thursday that the Kremlin would help move Navalny abroad if necessary and wished him a "speedy recovery".

Navalny has made a name for himself by exposing official corruption, labelling Putin's United Russia as "the party of crooks and thieves", and has served several jail terms.