Moscow (AFP) - Russia on Friday denied agreeing to free Nadia Savchenko, a Ukrainian military pilot who has been on hunger strike for two months and become a symbol of resistance at home.

President Petro Poroshenko raised the fate of Savchenko at this week's Minsk summit with Kremlin strongman Vladimir Putin and the leaders of Germany and France.

The pilot, who is being drip-fed glucose and vitamins, has lost over 20 kilogrammes (3 stone) since going on strike on December 13 in protest at her detention.

Poroshenko said he had pushed for Savchenko's immediate release and that German Chancellor Angela Merkel and French President Francois Hollande had backed him up.

Russian officials, however, said Savchenko was not a "hostage" and her case would have to be decided in court.

Putin's spokesman Dmitry Peskov said that while the peace deal signed in Minsk on Thursday called for both sides to exchange all hostages and those "unlawfully detained", Savchenko did not qualify as such.

"The president repeated his position, that has been voiced many times before, that in this case, she is under investigation and the degree of her guilt or innocence will be established by a court," Peskov was quoted as saying by the state RIA Novosti news agency.

- Fight until the end -

The Investigative Committee, which reports directly to Putin, said on Friday that "rumours spread about some upcoming release of Nadezhda Savchenko are without grounds."

The committee said there was no reason to lift Savchenko's detention since it had "undisputable proof" of her guilt, adding that "it is for a court in the future to decide the degree of her guilt and the punishment."

"Unlike some politicians and so-called attorneys, we are guided by facts and not emotions or political tastes," it added.

Poroshenko had said Thursday that he pressed Putin in Minsk on Savchenko's case and was told she would be freed "shortly" after medical tests and a preliminary investigation.

Savchenko, 33, is charged with involvement in the deaths of two Russian reporters killed in east Ukraine in June and is being investigated by the FSB security service, successor to the Soviet-era KGB.

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She denies the charges.

Her lawyer Mark Feigin said Savchenko, who has entered her third month of hunger strike, was in a "buoyant mood" on Friday.

"Her weight has started to fall faster, now it's 58 kilograms (9 stone, 2 pounds). They say she is losing half a kilo a day," he told AFP.

Another of her lawyers, Nikolai Polozov, said that despite the drip-feed of glucose and vitamins "essentially her hunger strike is approaching a phase when irreversible changes start in her body."

"She's ready to fight until the end, that is until her death literally," he told AFP.

"Since Putin personally spoke about Savchenko at the Minsk summit then he is responsible for this. And if she dies in prison, the responsibility will be on him."

- 'Turning into martyr' -

Savchenko, whose detention was this week extended until May 13, is alleged to have been abducted by pro-Russian rebels in eastern Ukraine and smuggled to Moscow, where she is now being detained.

Russia says she crossed the border of her own accord, pretending to be a refugee.

In Ukraine, Savchenko in October won a seat in parliament, standing in absentia for the party of ex-prime minister Yulia Tymoshenko.

Vedomosti, a liberal business daily in Russia, said this week the Ukrainian pilot was quickly turning into a "martyr."

"The longer Savchenko is on hunger strike, the more chances she has to become a new symbol of the arbitrariness of the Russian judical system," it said in an editorial.