The speaker of Russia's lower house of parliament has rejected a request from the president of the Parliamentary Assembly of the Council of Europe (PACE) to help secure the release of Nadiya Savchenko, a Ukrainian military pilot who is jailed in Russia.

PACE President Anne Brasseur appealed to State Duma speaker Sergei Naryshkin last week to help free Savchenko, whose health is deteriorating more than a month into a hunger strike she has vowed to maintain until she is released or dies.

But Naryshkin, speaking on January 25 after a meeting with Brasseur, said Savchenko's fate was up to Russia's courts.

Naryshkin said, "Only an investigation and the court can establish guilt; and if guilt is not proven, then she will be freed."

Critics say Russian courts do the Kremlin's bidding.

Savchenko has been charged with involvement in the deaths of two Russian journalists killed in the conflict in eastern Ukraine.

Savchenko, who was captured by pro-Russian separatists in Ukraine in June and transferred to Russian custody in July, says Russia has no right to hold or try her.

Savchenko won a seat in Ukraine's parliament in October, and party leader Yulia Tymoshenko said in November that Savchenko would be one of Ukraine's representatives in PACE.

Based on reporting by TASS and Interfax