North Korea said on Thursday that it had sentenced a U.S. citizen to 15 years of prison labor for carrying out hostile acts against the government in Pyongyang.

Pae Jun-Ho, known in the U.S. as Kenneth Bae, is a Korean-American tour operator who lived in state of  Washington and traveled with a group of businessmen in November last year from Yanji, China, to the North Korean Special Economic Zone of Rason, where he was arrested.

The sentence comes at a time of simmering tensions between Washington and Pyongyang.

“The Supreme Court sentenced him to 15 years of compulsory labor for this crime,” Pyongyang’s official Korean Central News Agency (KCNA) said.

Earlier, KCNA said that during investigations, Bae “admitted his criminal plot to overthrow our republic out of hostility,” and that his crime was “clearly substantiated by evidence.”

Seoul-based activist Do Hee-Yoon told the AFP that he suspected Bae was arrested because he had taken photographs of emaciated children in North Korea as part of efforts to appeal for more outside aid.

The U.S. State Department said the welfare of U.S. citizens was a “critical and top priority” and called on North Korea to release Bae immediately “on humanitarian grounds.”

“The Embassy of Sweden in Pyongyang acts as our protecting power for issues involving U.S. citizens in North Korea, so we are in close coordination with representatives at the Embassy of Sweden, and we understand they were last able to visit this U.S. citizen on Friday, April 26,” Acting Deputy Spokesperson Patrick Ventrell said on Monday.

A U.S. official who spoke to Reuters on condition of anonymity said on Monday that Bae had entered North Korea with a valid visa and that the U.S. did not want his case exploited for political gain.

“In the past there have been many instances where American citizens are used as political bargaining chips, and our concern is that this individual not be used in that manner,” the U.S. official said.

Bill Richardson, a former New Mexico governor and a former U.S. ambassador to the U.N, was unable to meet Bae or secure his release when he visited North Korea in January with Google chairman Eric Schmidt.