PYONGYANG, North Korea — North Korea's highest court sentenced an American tourist to 15 years in prison with hard labor for subversion on Wednesday, weeks after authorities presented him to media and he tearfully confessed that he had tried to steal a propaganda banner.

Otto Warmbier, 21, a University of Virginia undergraduate, was convicted and sentenced in a one-hour trial in North Korea's Supreme Court.

The court held that he had committed a crime “pursuant to the U.S. government's hostile policy toward (the North), in a bid to impair the unity of its people after entering it as a tourist.”

North Korea regularly accuses Washington and Seoul of sending spies to overthrow its government.

Tensions are particularly high following North Korea's recent nuclear test and rocket launch, and massive joint military exercises underway between the U.S. and South Korea that the North sees as a rehearsal for invasion.

U.S. State Department spokesman Mark Toner said the sentence was “unduly harsh” and urged North Korea to pardon Warmbier and release him on humanitarian grounds.

Before the trial, Warmbier had said he tried to steal a propaganda banner as a trophy for an acquaintance who wanted to hang it in her church. He identified the church as Friendship United Methodist Church in Wyoming, Ohio, where his family still lives.

Ohio Gov. and Republican presidential hopeful John Kasich issued a statement Wednesday calling on North Korea to immediately release Warmbier.

Bill Richardson, a former U.S. ambassador to the United Nations, said he met with North Korean diplomats in New York on Tuesday to request Warmbier's release after the student's parents and Kasich asked him to intervene. Richardson said he was neither encouraged nor discouraged by the meeting.