SEOUL, South Korea - The United States demanded Thursday that North Korea immediately release an American sentenced this week to 15 years of hard labor on charges of trying to overthrow the government.

The Obama administration is calling for amnesty for Kenneth Bae, State Department spokesman Patrick Ventrell said.

"What we're urging the DPRK authorities to do is to grant him amnesty and to allow for his immediate release, full stop," Ventrell said, using the acronym for North Korea's formal title, the Democratic People's Republic of Korea.

In a brief statement released Thursday by its state-run news agency, the North said its Supreme Court had handed down the sentence for Bae, a tour operator from Washington state, on Tuesday.

Bae's punishment complicates decision-making for Washington, which had hoped to open talks with North Korea if it showed signs of curbing its weapons program. The North has detained six Americans since 2009, using them in some cases to leverage rescue trips from former Presidents Jimmy Carter and Bill Clinton.

Ventrell did not rule out another such trip, although Carter's office said he had no plans to go.

In an apparent attempt to appeal to Pyongyang, the State Department did not flatly condemn Bae's trial as a sham, saying only that it had concerns about the transparency and fairness of the legal system in North Korea, and it did not call for the verdict to be nullified.

Before Thursday's announcement, the State Department had appealed for Bae's release on humanitarian grounds.

Bae, 44, was arrested in November while traveling with a small group in Rason, a special economic zone. The North has provided few specifics about his actions there, but activists in Seoul speculate he was perhaps found with pictures of hungry children.

Euna Lee, a journalist detained by the North in 2009, wrote in a Facebook post that Bae "guided a tour group to North Korea and was stopped by the authorities for some files on his computer hard drive that he wasn't aware of."

Bae's sentencing follows a tense period during which Pyongyang, under new leader Kim Jong Un, tested an underground nuclear device, nullified an armistice agreement that ended the Korean War, and pulled its workers from an industrial complex jointly operated with the South.

Also Thursday, the Pentagon said in a report to Congress that North Korea appears to be on track to fulfill its stated goal of being able to strike the United States with a nuclear-armed missile.

The report said the North's efforts had been greatly aided by its work on a space-launch vehicle, highlighted by the launch of a satellite into space in December. But it added that the North has yet to test a reentry vehicle, needed for delivering a warhead to a target.