A Ukrainian military officer imprisoned in Russia has turned 34 behind bars amid renewed international clamor for her to be released.

Supporters of Nadezhda Savchenko urged social media users to use the hashtag #FreeSavchenko to mark her birthday on Monday. The U.S. ambassador to Ukraine used the hashtag on Twitter and posted a picture of a birthday card addressed to Savchenko.

Savchenko has been in Russian custody since June on charges that she aided a mortar attack that killed two Russian journalists covering the war between Ukrainian troops and separatists. She denies involvement in the reporters' deaths.

Ukraine has demanded Savchenko be released in line with a February cease-fire agreement requiring all prisoners captured during the war in Ukraine to be released. Moscow says Savchenko does not qualify for release.