* Sentencing follows months of nuclear tensions and threats
* North Korea has used U.S. prisoners as negotiating assets
By Ju-min Park and Paul Eckert
SEOUL/WASHINGTON, May 2 (Reuters) - North Korea sentenced an American citizen to 15 years of hard labor on Thursday for what it said were crimes against the state, prompting the United States to call for his immediate release to keep him from becoming a bargaining chip between the two countries.
Kenneth Bae, 44, was born in South Korea but is a naturalized U.S. citizen and studied psychology for two years at the University of Oregon. His sentencing comes after two months of saber-rattling that saw North Korea threaten the United States and South Korea with nuclear war.
Pyongyang has previously tried to use American prisoners as negotiation assets in talks with Washington. Washington is not looking for an envoy to try to secure Bae's release as it has sometimes done, said a U.S. official who spoke on condition of anonymity.
The official said the United States has sought in recent years to break out of a pattern of having to resolve repeated crises with North Korea through transactional deals.
"We urge the DPRK (North Korea) to grant Mr. Bae amnesty and immediate release," State Department deputy spokesman Patrick Ventrell said. It was not clear if Bae had been taken immediately to jail.
Bruce Klingner, a former CIA North Korea analyst, dismissed the idea that Bae's release would trigger the renewal of long-stalled diplomacy.
"Previous arrests of U.S. citizens didn't lead to changes in North Korean policy, resumption of bilateral dialogue or breakthroughs in U.S.-North Korean relations," said Klingner, a senior fellow at Washington's Heritage Foundation think tank.
Human rights activists in South Korea say Bae may have been arrested for taking pictures of starving children.
Bae is "a committed Christian," said David Ross, director of a missionary training center at Antioch World Ministries Inc in Monroe, Washington.
"He has feelings for orphans and has done some ministry work feeding orphans," added Ross, who said he has been a casual acquaintance of Bae since they met four years ago through church affiliations in Hawaii. "He has a missionary heart," Ross said.
Bae's mother, Myung-Hee Bae, told Reuters in a brief telephone interview that she had last spoken to her son on April 23 and that he told her during the call that he was well. Myung-Hee Bae, who lives in the Seattle area, said it was the only call she had received from her son since he was detained.
His sister, Terri Chung, told CNN in an interview that he had been working as a tour guide bringing people from China into North Korea and that he had never before run into trouble doing so.
"We can't really know for sure why he would be arrested. He's only had the biggest heart for the people and the nation of North Korea," she said. "We are baffled just like anybody else about why a man like my brother could be arrested."
"We just pray and ask for leaders of both nations to please just see him as one man caught in between and we just ask that he be allowed to come home," Chung said.
Bae was one of five tourists who visited the northeastern North Korean city of Rajin in November and has been held since then. The State Department recommends that U.S. citizens avoid travel to North Korea, although it does not block trips.
"U.S. citizens crossing into North Korea, even accidentally, have been subject to arbitrary arrest and long-term detention," reads the department's travel warning, updated in March.
Bill Richardson, a former U.S. ambassador to the United Nations who has made numerous trips to North Korea that included efforts to free detained Americans, said Bae's case should not become entangled in the current U.S.-North Korea impasse.
"Now that the sentencing and the North Korean legal process has been completed, it is important that negotiations begin to secure Kenneth Bae's release on humanitarian grounds or a general amnesty," said Richardson, who visited North Korea in January with Google Inc CEO Eric Schmidt.
White House spokesman Jay Carney said any negotiations with North Korea were "dependent upon the North Koreans demonstrating a willingness to live up to their international obligations."
North Korea is the subject of U.N. Security Council resolutions calling for an end to its nuclear and missile tests, as well as punitive U.N. sanctions.
Some media reports have identified Bae as the leader of the tour group. NK News, a specialist North Korea news website, said he was the owner of a company called Nation Tours that specialized in tours of northeastern North Korea.
The reports could not be verified and North Korean state news agency KCNA did not list any specific charge other than crimes against the state, and used a Korean rendering of Bae's name, Pae Jun-ho, when it reported the Supreme Court ruling.
"North Korea has shown their intention to use him as a negotiating card as they have done in the past," said Cheong Seong-chang, senior fellow at the Sejong Institute, a think tank in Seoul.
North Korea appears to use the release of high-profile American prisoners to extract a form of personal tribute, rather than for economic or diplomatic gain, often portraying visiting dignitaries as paying homage.
Former U.S. President Jimmy Carter, who has traveled to North Korea before to try to free a detained American, has no plans to do so for Bae, Carter's spokeswoman said.
According to North Korean law, the punishment for hostile acts against the state is between five and 10 years' hard labor.
"I think his sentencing was hefty. North Korea seemed to consider his acts more severe," said Jang Myung-bong, honorary professor at Kookmin University in Seoul and a North Korea law expert.
North Korea is one of the most isolated states on earth. Its official policy of "Juche," or self-reliance, is a fusion of Marxism, extreme nationalism and self-sufficiency centered on the cult of the ruling Kim family.
Bae likely will not be incarcerated in one of the North's notorious slave labor camps, such as the one where defector Kwon Hyo-jin was locked up. There, Kwon said, prisoners were worked to death and often survived only by eating rats and snakes.
"If an American served jail together with North Korean inmates, which won't happen, he could tell them about capitalism or economic developments. That would be the biggest mistake for North Korea," said Kwon, a North Korean sentenced to a camp for seven years until 2007. He defected to South Korea in 2009.
"(Bae) would be sent to a correctional facility that only houses foreigners and was set up as a model for international human rights groups."
Bae's sentencing brought bad back memories for Euna Lee, one of two U.S. journalists sentenced to 12 years in 2009 and released only after a visit to Pyongyang by former President Bill Clinton. She cried out loud in the courtroom when her labor camp sentence was handed down.
"The word 'labor camp' took the tiny hope I had away from me. I was physically and mentally weak and I really thought I would not make it home," South Korea-born Lee said via email.
Lee, then a journalist for Current TV, said her 12-year sentence included two years for illegal border crossing and 10 for the "hostile act" of making a documentary on North Koreans who risk their lives fleeing their country for nearby China.
Bae was given counsel by the Swedish Embassy in Pyongyang because the United States does not have diplomatic relations with North Korea. The embassy has declined to comment on the case and Ventrell said the Swedes did not attend Bae's trial.