PYONGYANG, Korea, Democratic People's Republic Of -- North Korea's highest court sentenced an American tourist to 15 years in prison with hard labour 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.

He was charged with subversion under Article 60 of North Korea's criminal code. 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 to enable the U.S.-backed South Korean government to take control of the Korean Peninsula.

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

"Despite official claims that U.S. citizens arrested in the DPRK are not used for political purposes, it's increasingly clear from its very public treatment of these cases that the DPRK does exactly that," Toner told reporters, referring to the North by its official name, the Democratic People's Republic of Korea.

The University of Virginia said it was aware of news reports about Warmbier and remained in touch with his family, but would have no additional comment at this time.

A message seeking comment from Warmbier's family was left at a Wyoming, Ohio, telephone listing for his father, Fred Warmbier. Susanna Max, a spokeswoman for Wyoming City Schools, said last month that the district, where Otto Warmbier attended school, had been in touch with the family. She said Wednesday that the district continues "to respect their privacy" and declines to comment.

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. That would be grounds in North Korea for a subversion charge. He identified the church as Friendship United Methodist Church. Meshach Kanyion, pastor of the church in Wyoming, declined to comment Wednesday.

Ohio Gov. and Republican presidential hopeful John Kasich issued a statement Wednesday calling on North Korea to immediately release Warmbier and let him return to his family. "His detention was completely unjustified and the sentence North Korea imposed on him is an affront to concepts of justice," Kasich said.

Bill Richardson, a former U.S. ambassador to the United Nations, said he had 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 breakfast meeting with the diplomats from the North Korean mission to the U.N. They said they would relay his request to Pyongyang.

Richardson said based on past experience, North Korea could release Warmbier after sentencing, but current U.S.-North Korean tensions could hurt those prospects.

"My concern now is that the U.S.-North Korean relationship is in very low, negative ebb, and I hope that does not affect a humanitarian negotiation for the release of Otto," Richardson told The Associated Press.

Trials for foreigners facing similar charges in North Korea are generally short and punishments severe. Warmbier was arrested as he tried to leave the country in early January. He was in North Korea with a New Year's tour group.

U.S. tourism to North Korea is legal. Arrests of tourists are rare but the U.S. State Department strongly advises against such travel.

Further complicating matters, Washington and Pyongyang do not have diplomatic relations. The Swedish Embassy in Pyongyang acts as a go-between in consular issues when U.S. citizens run afoul of North Korean authorities.

North Korea announced Warmbier's arrest in late January, saying he committed an anti-state crime with "the tacit connivance of the U.S. government and under its manipulation." It remains unclear how the U.S. government was allegedly connected to Warmbier's actions.

Warmbier had been staying at the Yanggakdo International Hotel. It is common for sections of tourist hotels to be reserved for North Korean staff and off-limits to foreigners.

In a tearful statement made before his trial, Warmbier told a gathering of reporters in Pyongyang he was offered a used car worth $10,000 if he could get a propaganda banner and was also told that if he was detained and didn't return, $200,000 would be paid to his mother in the form of a charitable donation.

Warmbier said he accepted the offer because his family was "suffering from very severe financial difficulties."

Warmbier also said he had been encouraged by the university's "Z Society," which he said he was trying to join. The magazine of the university's alumni association describes the Z Society as a "semi-secret ring society" founded in 1892 that conducts philanthropy, puts on honorary dinners and grants academic awards.

In previous cases, people who have been detained in North Korea and made a public confession often recant those statements after their release.

In the past, North Korea has held out until senior U.S. officials or statesmen came to personally bail out detainees, all the way up to former President Bill Clinton, whose visit in 2009 secured the freedom of American journalists Euna Lee and Laura Ling.

In November 2014, U.S. spy chief James Clapper went to Pyongyang to bring home Matthew Miller, who had ripped up his visa when entering the country and was serving a six-year sentence on an espionage charge, and Korean-American missionary Kenneth Bae, who had been sentenced to 15 years for alleged anti-government activities.

Jeffrey Fowle, another U.S. tourist from Ohio detained for six months at about the same time as Miller, was released just before that and sent home on a U.S. government plane. Fowle left a Bible in a local club hoping a North Korean would find it, which is considered a criminal offence in North Korea.

Eric Talmadge, the AP's Pyongyang bureau chief, contributed to this report from Tokyo. AP writers Lisa Cornwell in Cincinnati, Ohio, and Sarah Brumfield and Matthew Pennington in Washington, D.C., also contributed to this story.