Russian investigators say there are no grounds to release a key Ukrainian prisoner despite claims by Ukraine's president that she would soon be freed.

In a statement posted online Friday, Russia's Investigative Committee said that only a court could free Nadezhda Savchenko, a Ukrainian air force officer who has been on hunger strike in a Moscow prison for more than two months.

After marathon peace talks in Minsk, Belarus, that ended on Thursday, Ukrainian President Petro Poroshenko declared that Savchenko would soon be released.

Russia has charged her with involvement in the deaths of two Russian journalists in eastern Ukraine. Russia claims Savchenko voluntarily crossed the border into Russia before she was detained, which she denies.

The Minsk agreement calls for the exchange of all illegally held prisoners and hostages.