A Ukrainian air force pilot who was captured by separatist rebels last month has been charged in Russia with abetting the killing of two Russian journalists, Russian officials have said.

Ukraine has been rocked by fighting between pro-Russia rebels and government forces for more than three months.

Igor Kornelyuk and Anton Voloshin, employees of a Russian state-owned TV channel, died in June after being hit by mortar fire while on assignment in the Ukrainian city of Luhansk.

Russia's Investigative Committee said in a statement on Wednesday that 31-year-old pilot Nadezhda Savchenko is suspected of tipping off Ukrainian troops as to the whereabouts of the journalists as well as other unspecified "civilians" who were in a rebel-held area.

The investigators said Savchenko was a Ukrainian officer, but she went on leave earlier this summer to join a voluntary battalion.

Savchenko is now in a Russian detention centre but it is unclear how she got there, the AP news agency reported.

Moscow says she crossed the border voluntarily, without documents and under the guise of a refugee.

They said they have yet to find out why she went to Russia.

Ukraine, however, recently raised the alarm about her disappearance, with the foreign ministry issuing a protest note.