A senior U.S. envoy who will travel to North Korea later this week said Wednesday that he plans to strongly appeal for the release of an American sentenced to 15 years of hard labor by the authoritarian state but added that Washington has received no guarantees from Pyongyang the ailing man will be freed.

Bob King, the U.S. special envoy for North Korean human rights issues, said during a stopover in Tokyo that the United States is increasingly concerned about the health of 45-year-old Kenneth Bae, a tour operator and Christian missionary who was arrested last November and accused of committing ``hostile acts'' against North Korea.

``We're going to make an appeal,'' King said after a meeting with Japanese officials. ``He has health problems and we're hopeful we will be able to make progress on that.''

When asked if he was confident Bae would be released, he said. ``We haven't been told that anything is definite.''

King will fly to Pyongyang from a U.S. military base near Tokyo on Friday, and fly back on Saturday.

It will be the first public trip to North Korea by a U.S. administration official in more than two years. The U.S. has requested a pardon and amnesty on humanitarian grounds for Bae, who suffers multiple health problems and was recently hospitalized. Washington has been calling for Pyongyang to grant amnesty since Bae was sentenced on April 30.

A breakthrough on the Bae case could provide an opening for an improvement in relations severely strained by Pyongyang's pursuit of nuclear weapons. Two senior Obama administration officials reportedly made secret visits to North Korea in 2012 in an effort to improve relations with the government of young leader Kim Jong Un, but apparently made little headway.

North Korea has previously used detained Americans as bargaining chips in its standoff with the U.S. over its nuclear and missile programs. Multination aid-for-disarmament talks have been on hold since 2009, and efforts by Washington to negotiate a freeze in the North's nuclear program in exchange for food aid collapsed 18 months ago.

In Washington, State Department spokeswoman Marie Harf said Wednesday that the U.S. was hopeful that King would return with Bae, but she would not link that issue with the prospects for renewed U.S.-North Korean dialogue. Harf reiterated the U.S. position that North Korea should honor its previous commitments to abandon nuclear weapons.

Bae is at least the sixth American detained in North Korea since 2009. The others were eventually allowed to leave without serving out their terms, some after prominent Americans, including former Presidents Bill Clinton and Jimmy Carter, visited North Korea.

If North Korea releases Bae, it will be the second time King has come back with a prisoner.

When he last visited North Korea in May 2011 to assess the impoverished North's food situation, he came home with Eddie Jun. Jun, a Korean-American from California, was arrested for alleged unauthorized missionary work during several business trips to the country. He was released on humanitarian grounds.

Bae's sister revealed earlier this month that he was moved from a labor camp to a hospital after losing more than 50 pounds (22 kilograms). Terri Chung, of Edmonds, near Seattle, says her brother, a father of three, suffers from diabetes, an enlarged heart, liver problems and back pain.

According to U.S. officials, Washington first made its offer to send King to North Korea several weeks ago, but Pyongyang only recently took them up on it.

North Korea and the U.S. do not have formal diplomatic relations. Relations went into a tailspin after Pyongyang conducted long-range rocket launches and a February nuclear test in defiance of U.N. Security Council resolutions. But North Korea has dialed down its rhetoric in recent months and has moved to improve its relations with rival South Korea, a staunch U.S. ally.

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Associated Press writers Matthew Pennington and Matthew Lee in Washington contributed to this report. (AP)