Politicians and human rights groups are calling on the North Korean government to release a 21-year-old U.S. tourist sentenced Wednesday to 15 years of hard labor for trying to steal a propaganda sign.

North Korean authorities detained Otto Warmbier, 21, at the Pyongyang Sunan International Airport on Jan. 2, one day after he tried to take a sign at the Yanggakdo International Hotel in Pyongyang. Officials said he was accused of committing a "hostile act."

During a press briefing Wednesday, White House Press Secretary Josh Earnest accused the hermit kingdom of using American citizens as "pawns to pursue a political agenda."

"We strongly urge the North Korean government to pardon him and grant him special amnesty, and immediately release him," he said.

In a tearful confession televised on state-run media in January, Warbier characterized his attempt to take the propaganda sign as a "severe" criminal act "aimed at harming the work ethic and the motivation of the Korean people."

"I have been very impressed by the Korean government's humanitarian treatment of severe criminals like myself and of their very fair and square legal procedures," he said.

It was not clear whether the confession was made of his own volition or coerced.

Republican presidential candidate and Ohio Gov. John Kasich also called for Warmbier's release in a statement Wednesday. Warmbier attends the University of Virginia and is originally from Ohio.

"His detention was completely unjustified and the sentence North Korea imposed on him is an affront to concepts of justice," he said. "Continuing to hold him only further alienates North Korea from the international community. I urge the Obama Administration to redouble its efforts to secure his release and ask all Ohioans to continue to lift up Otto and his family in prayer in support of his swift, safe return."

Warmbier said he tried to take the sign for a member of an Ohio church and with the encouragement of the University of Virginia's secretive Z Society, according to the New York Times. The church member, who was not identified, reportedly offered to buy Warmbier a used car worth $10,000 in exchange for the sign. The Z Society allegedly promised Warmbier entry into the society "if he helped to promote the group's goal of spreading freedom and eliminating tyranny," the New York Times reported.

The Human Rights Watch added its name to the list of those criticizing North Korea for the Warmbier case in a statement Wednesday.

"North Korea's sentencing of Otto Warmbier to 15 years hard labor for a college-style prank is outrageous and shocking," said Phil Robertson, deputy Asia director for the Human Rights Watch.

Warmbier's sentence came down amid heightened tensions between North Korea and the United States. The U.S. recently condemned the country for its nuclear weapons test on Jan. 6 and its ballistic missile technology test on Feb. 7.

On Wednesday, President Barack Obama announced tougher sanctions against North Korea, due in part to the recent tests.

"These actions are consistent with our longstanding commitment to apply sustained pressure on the North Korean regime," Earnest said.