Jailed Ukrainian pilot Nadiya Savchenko has resumed her hunger strike and is withering away, her lawyer said on Monday, adding that authorities planned to transfer her to a Russian civilian hospital.

Lawyer Nikolai Polozov said that Savchenko, who had already lasted for more than 80 days without food before halting her hunger strike last month, has again been refusing food since last week.

According to a leading human rights official, she has lost eight kilogrammes in almost as many days and "her tests are very poor".

The Ukrainian military flyer, who will be 34 next month, first went on hunger strike in December to protest her arrest last year for alleged involvement in an attack that killed two Russian journalists on eastern Ukraine's frontlines.

She had enlisted as a volunteer in one of the battalions fighting pro-Russian rebels and has become a national hero to many Ukrainians since her capture.

Last week Russia charged Savchenko, who is also a Ukrainian lawmaker, with acting as an accessory in the journalists' murder and with illegally crossing the border into Russia.

"Last week I noticed that she had lost weight, had become more gaunt than usual," Polozov told AFP.

"It has gradually become clear that she has been refusing food since last week, she now weighs a little over 50 kilogrammes," he said, adding that prison officials had known that Savchenko had resumed her hunger strike.

"They knew that she'd been refusing food but they did not inform us and she also did not say anything."

Polozov said Savchenko would on Tuesday be transferred to a civilian hospital in Moscow, suggesting that the prison service, known under its Russian acronym FSIN, wanted to absolve itself of any responsibility.

"I believe the main task that will be given them will be to try to save her life."

He said her upcoming transfer reminded him of the case of the jailed Soviet dissident Anatoly Marchenko who in 1986 was released from prison after refusing food for more than 100 days and who died soon afterwards.

"I interpret the actions of FSIN as an attempt to exonerate themselves from responsibility for what happens next."

- 'My only weapon' -

In March, Savchenko broke off her hunger strike over apparent concerns about her quickly deteriorating health and the failure of her protest to win her freedom, her lawyers said at the time.

She had consented to being given vitamins by drip.

"Hunger is my only weapon in the fight against the outrageous actions of the Russian authorities!" Savchenko said in a letter posted by Mark Feigin, another lawyer, on Twitter.

"So don't ask me to lay it down."

A member of the Kremlin human rights council, Elizaveta Glinka, said that Savchenko had lost eight kilogrammes over the past week.

"She does not eat, drinks only a little and her tests are very poor," Glinka said in a statement.

Ukrainian authorities said they were aware of the transfer.

"Indeed she needs to be hospitalised now," Yevgen Perebyinis, a spokesman for the Ukrainian foreign affairs ministry, told AFP.

Western leaders have repeatedly asked Putin to release Savchenko, who has received the Hero of Ukraine award, the ex-Soviet country's highest honour.

The Ukrainian pilot will be transferred to a civilian hospital just as Russia is gearing up to celebrate the 70th anniversary of the Soviet victory over Nazi Germany and expects German Chancellor Angela Merkel to visit Moscow on May 10.

After a summit Monday between top European Union officials and Ukraine, European Council President Donald Tusk called on Russia to stick to a peace deal agreed in February.

"We expect the urgent release of all hostages, including Nadiya Savchenko," he added.