Russia has come under pressure from Europe's human rights watchdog and the United States to free Nadezhda Savchenko, a Ukrainian air force officer who is on hunger strike in a Russian prison.

Savchenko is awaiting trial on charges of involvement in the killing of two Russian journalists during the conflict in eastern Ukraine. Her lawyer says she has been on a hunger strike since Dec. 13.

The Parliamentary Assembly of the Council of Europe on Wednesday called on Russia to release her "within 24 hours and to ensure her return to Ukraine or to hand her over to a third country."

State Department spokeswoman Jen Psaki said the U.S. also was calling on Russia to release Savchenko immediately and was concerned by reports that she was gravely ill.