The U.S. government called Thursday for the immediate release of a Korean-American man sentenced by North Korea to 15 years of forced labor for unspecified "crimes" against Pyongyang.

"What we're urging the DPRK (North Korean) authorities to do is to grant him amnesty and to allow for his immediate release,"

Patrick Ventrell, deputy spokesman for the State Department, said at a press briefing.

He took issue with the lack of transparency in the communist nation's legal system.

"We've had deep concerns about the transparency and due process across the breadth of the North Korean legal system," he added.

The North earlier announced that its highest court sentenced Kenneth Bae, whose Korean name is Bae Joon-ho, to 15 years of compulsory labor for committing crimes against the nation. A brief dispatch by the North's official news agency, KCNA, gave no specifics.

Bae, a U.S. citizen based in China, was arrested in North Korea in November after entering the nation with a group of tourists.

Some reports say he was serving as a tour operator, while others say he worked as a Christian missionary.

Ventrell repeatedly used the word "amnesty," but he did not elaborate on whether the U.S. accepts the North's ruling itself.

"There is no greater priority than the safety of U.S. citizens," Ventrell said. "Now that Mr. Bae has gone through a legal process, we urge the DPRK to grant him amnesty."

The U.S. government has been trying to win the release of Bae through the Swedish Embassy in Pyongyang. It has acts as protecting power for U.S. citizens in North Korea. The U.S. has no diplomatic office in the North since they have no formal diplomatic ties.

The embassy last had access to Bae on Friday, Ventrell said.

Asked if the U.S. believes Bae is innocent, he said, "We don't know the facts of the case."

Several American citizens had been detained in North Korea on similar charges, but all were freed, largely unhurt.

In 2009, former U.S. President Bill Clinton flew to Pyongyang to bring home two female American journalists.

The following year, former U.S. President Jimmy Carter won plaudits when he negotiated the release of American national Aijalon Mahli Gomes.

Ventrell said he is not aware of any attempt to resolve the Bae case through the dispatch of such a high-profile envoy.

South Korean media reports said Carter may play a role again.

But his aides said he has no plans to visit North Korea.

"President Carter has not had an invitation to visit North Korea and has no plans to visit," Carter's press secretary, Deanna Congilio, said by email. (Yonhap News)