WASHINGTON (AFP) - The United States on Thursday called for the "immediate release" of a Korean-American tour operator sentenced in North Korea to 15 years of hard labour for "hostile acts" against the government.

"What we're urging the DPRK authorities to do is to grant him amnesty and to allow for his immediate release, full stop," deputy acting State Department spokesman Patrick Ventrell told journalists.

Mr Pae Jun Ho, who is known in the United States (US) as Kenneth Bae, was arrested in November as he entered the northeastern port city of Rason. He has been accused of trying to "topple the DPRK" (North Korea).

"The Supreme Court sentenced him to 15 years of compulsory labour for this crime," according to the North's official Korean Central News Agency (KCNA), which said his trial was held on April 30.

"There hasn't been transparency in the case," Mr Ventrell added, saying Washington still did not "know all of the facts" and was seeking details of the charges filed against Mr Bae.

Mr Kim Jong Un's isolated regime is likely to use the detainee as a bargaining chip, experts said, as it seeks concessions from the US following weeks of bellicose threats of missile strikes and of nuclear war.

Several Americans have been held in North Korea in recent years, and been freed after the visits of high-profile US leaders such as former presidents Jimmy Carter and Bill Clinton.

Mr Ventrell said he would not "characterise" Mr Bae as being a political pawn, but sidestepped questions about whether the US was thinking of sending an envoy to Pyongyang to press for his release.

A Carter spokesman also said the former president has no plans to travel to the isolated nation. "President Carter has not had an invitation to visit North Korea and has no plans to visit," spokesman Deanna Congileo said.

Pyongyang has not specified the basis of the offenses allegedly committed by Mr Bae, who is reported to be aged 44, but KCNA has previously said that he admitted his crimes.

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

Officials from the Swedish embassy, which acts as America's protecting power with Pyongyang due to the lack of diplomatic relations, had not attended the trial, Mr Ventrell said, and last visited Mr Bae in jail on April 26.

"Fifteen years of hard labour is something we're pretty concerned about," said a senior US official, who asked not to be named, adding that Washington was for now continuing to press through the Swedes for more information.

US politician Bill Richardson failed to secure Mr Bae's release when he visited North Korea in January with Google chairman Eric Schmidt.

Tensions have been running high between the US and North Korea since Pyongyang carried out a third nuclear test in February.

The North reacted furiously to the use of nuclear-capable B-52s and B-2 stealth bombers in recent joint US-South Korean military drills.

White House spokesman Jay Carney said Washington had "made it clear that there is a path open to the North Koreans that would allow for negotiations."

But he stressed the North had to demonstrate a "willingness to live up to their international obligations."

In 2011, a US delegation secured the release of Eddie Jun Yong Su, a California-based businessman held for apparent missionary activities.

In 2010, Mr Carter won plaudits when he negotiated the release of American national Aijalon Mahli Gomes, sentenced to eight years of hard labour for illegally crossing into the North from China.

And on another mercy mission a year earlier in 2009, Mr Clinton managed to free US television journalists Laura Ling and Euna Lee, jailed after crossing the North Korean border with China.

Experts believe the North is likely to try to use Mr Bae to extract concessions from Washington.

"But the whole atmosphere is quite different from when similar hostage disputes erupted in the past," said Mr Yang Moo Jin, a professor at the University of North Korean Studies in Seoul.