WASHINGTON, March 16 (Xinhua) -- The United States on Wednesday accused the Democratic People's Republic of Korea (DPRK) of holding an American student for political reasons and called for his immediate release.

Pyongyang is using U.S. citizens as "pawns to pursue a political agenda," White House spokesman Josh Earnest said.

Otto Warmbier, 21, a student at the University of Virginia, was sentenced to 15 years of hard labor by DPRK's Supreme Court earlier Wednesday.

Warmbier admitted in court that he took down a political slogan from the wall of the hotel where he stayed during a tour in Pyongyang in January.

The sentence of hard labor is being served starting from Feb. 4, and the ruling was unappealable, the DPRK's Supreme Court said.

"We strongly encourage the North Korean government to pardon him and grant him special amnesty and immediate release," said Earnest.

"The allegations for which this individual 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."

Also on Wednesday, U.S. State Department spokesman Mark Toner said the sentence was "unduly harsh" and urged DPRK to pardon and free Warmbier.

"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 in a daily new briefing.

The incident comes amid high tensions on the Korean Peninsula with the United States and its ally South Korea presently carrying out their largest-ever military drills, which will last till April 30.

U.S. tourism to DPRK is legal. However, the U.S. State Department strongly advises against such travel.

">