(Reuters) - The United States on Friday called on Moscow to immediately release a jailed Ukrainian military pilot who has been moved to solitary confinement.

State Department spokeswoman Jen Psaki said there were concerns about the health of the pilot, who is on a hunger strike.

"We're deeply concerned by reports that Russia has moved Ukrainian pilot Nadia Savchenko to solitary confinement," Psaki told a daily briefing. "We understand that Ms Savchenko has been on hunger strike for nearly a month to protest the terms of her detention and is suffering additional health problems.

"We call for her immediate release, as well as other Ukrainian hostages held by Russia," Psaki added.

Savchenko, 33, was captured by separatists in eastern Ukraine last June and sent to Russia where she had been held in a psychiatric clinic. Ukraine has said she is one of hundreds of prisoners being held by the pro-Russian separatists.