"The chief doctor stated that Navalny is not transportable. Condition is unstable," Kira Yarmysh said on Twitter.

"The ban on transporting Alexei is a direct threat to his life. It is deadly to remain in the Omsk Hospital without equipment or a diagnosis."

Yarmysh said an air ambulance dispatched to fly him to Germany for treatment was due to land shortly.

"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."

Navalny, a 44-year-old lawyer and anti-corruption campaigner who is among President Vladimir Putin's fiercest critics, was hospitalised in Omsk after he lost consciousness while on a flight and his plane made an emergency landing.

Doctors said they were working to save his life after he went into a coma and was put on a ventilator in intensive care.

There has been no official diagnosis of his condition, but his supporters believe he was poisoned because of his political activities.

Germany and France have offered to help and German media reported that the air ambulance had left Nuremberg early Friday to collect Navalny.

Navalny's team has said that the hospital in Omsk is ill-equipped and his doctor Anastasia Vasilyeva said she had asked for the Kremlin's help to transfer him to a European clinic.

Jacaranda FM/AFP