North Korea's highest court sentenced a detained U.S. student to 15 years of hard labor on crimes against the communist state, foreign news wire services reported Wednesday.

Otto Warmbier, a 21-year-old U.S. university student, was arrested and detained by the North in January this year for allegedly conducting hostile acts against the regime.

The court said the student attempted to steal a propaganda banner from a restricted area of his hotel, according to the Associated Press.

There is no official report on the rulings from the North's state media.

The North's Korean Central News Agency (KCNA) earlier said that the student entered North Korea nominally for tourism, though his real intention was to undermine North Korea's unity under the U.S. government's approval and control.

Warmbier told a press conference in Pyongyang on Feb. 29 that he stole a political sign that had promoted "the Korean people's love for their system" from the hotel.

The KCNA said he had confessed to his "severe" crime and asked for forgiveness.

In early March, the United Nations Security Council imposed tougher sanctions against North Korea following Pyongyang's nuclear test and long-range rocket launch.

Experts said the North has used detained Americans as leverage to force the U.S. to open bilateral talks with it.

In 2014, Pyongyang released three detained Americans -- Kenneth Bae, Matthew Todd Miller and Jeffrey Fowle.

Lim Hyeon-soo, a Korean-Canadian pastor, has also been held in captivity in the North since he entered the country via China on a humanitarian mission in January 2015.

In December, the North's highest court sentenced Lim to life in prison, citing his "subversive plots" against the North's regime. (Yonhap)