KYIV -- Ukrainian military pilot Nadia Savchenko has been placed in solitary confinement at a pretrial detention center in Moscow.

Savchenko's lawyer Mark Feigin told RFE/RL on January 9 that she was moved to solitary confinement due to the hunger strike she has been holding for almost a month.

Savchenko was captured by pro-Russian separatists in eastern Ukraine in June and jailed in Russia in July.

Russian authorities have charged her with complicity in the deaths of two Russian journalists who were killed covering the Ukraine conflict.

Savchenko denies the charges and says her transfer to Russia was illegal.

Feigin said that Savchenko, who underwent tests at a psychiatric facility against her will, has been found mentally fit to stand trial.

He said she had been under psychological pressure from investigators seeking to persuade her to plead guilty.

U.S. State Department spokeswoman Jen Psaki said on January 9 that Washington was concerned over the reports of Savchenko being put in solitary confinement.

She called on Russia to immediately release Savchenko, as well as other Ukrainian "hostages" it holds.

Savchenko's sister, Vira Savchenko, told RFE/RL that her sister was accepting glucose intravenously and drank tea on Orthodox Christmas Day on January 7.

She has been mainly ingesting warm water for 26 days.