The White House urged North Korea on Wednesday to release a detained U.S. college student after Pyongyang's highest court sentenced him to 15 years of hard labor for allegedly stealing a political propaganda sign.

White House press secretary Josh Earnest issued the appeal, accusing the North of using detained Americans as "pawns to pursue a political agenda." The verdict came as Pyongyang has been ramping up bellicose threats against the U.S. and South Korea.

The North's Supreme Court handed down the harsh verdict to the 21-year-old University of Virginia student, Otto Warmbier, on conviction of charges of state subversion. Warmbier has been detained in the communist nation since early January after getting caught stealing the sign from a hotel.

"Now that Mr. Warmbier has gone through this criminal process, we strongly urge the North Korean government to pardon him and grant him special amnesty and immediate release," Earnest said at a press briefing.

Earnest said that the allegations on which Warmbier was arrested and imprisoned "would not give rise to arrest or imprisonment in the United States or in just about any other country in the world."

"Despite official claims that U.S. citizens arrested in North Korea are not used for political purposes, it is increasingly clear that the North Korean government seeks to use these U.S. citizens as pawns to pursue a political agenda," he said.

Earnest also reiterated that there is no greater priority than the welfare and safety of U.S. citizens abroad.

"This underscores the risks associated with traveling to North Korea. And the Department of State strongly recommends against all travel by U.S. citizens to North Korea. And there's additional information about that on the State Department website," he said.

American visitors have often been detained in North Korea on charges of anti-state and other unspecified crimes. Widespread views are that the communist nation has used the detentions as bargaining chips in its negotiations with Washington.

Kenneth Bae, a Korean-American missionary, served two years of detention in the North before being released in November 2014, when U.S. Director of National Intelligence James Clapper made a secret trip to Pyongyang to win his release and that of another U.S. detainee.

Wednesday's sentencing came as tensions on the Korean Peninsula have been running high since the North's nuclear and missile tests. Pyongyang has also made a series of bellicose threats as the U.S. and South Korea conducted annual joint military exercises that the North has branded as a rehearsal for invasion.

State Department deputy spokesman Mark Toner also called for an immediate release of the detainee.

"The department believes that the sentence is unduly harsh for the actions Mr. Warmbier allegedly took," he said. "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."

The case only underscores the risks associated with travel to North Korea, he said.

"The Department of State strongly recommends against all travel by U.S. citizens to North Korea. Let me just repeat that again. The United States and the Department of State strongly recommend against all travel by U.S. citizens to North Korea," Toner said.

Meanwhile, former New Mexico Gov. Bill Richardson was quoted as saying he held a face-to-face meeting in New York on Tuesday with two North Korean diplomats stationed at Pyongyang's mission to the United Nations and called for an immediate release of the detained student.

Richardson, known for his roles in past dealings with Pyongyang, told The New York Times that he held the meeting with Jang Il-hun, deputy chief of the North's mission to the U.S. at the request of Republican presidential runner and Ohio Gov. John Kasich.

"I urged the humanitarian release of Otto, and they agreed to convey our request," Richardson was quoted as saying. He also said that Warmbier has been seen by a Swedish diplomat in Pyongyang. Sweden serves as protecting power for U.S. interests in the North. (Yonhap)