The U.S. human rights envoy for North Korea on Monday pressed for the release of a Korean-American detainee who has been held prisoner in the communist country for the last nine months.

Kenneth Bae, a U.S. citizen based in China, was traveling in North Korea in November when he was arrested by authorities on charges of trying to overthrow the communist regime. In April, he was sentenced to 15 years of hard labor.

"We've requested the North release Mr. Bae on humanitarian grounds. His health is suffering ... We hope they will listen to our request," Robert King told reporters in Seoul.

The envoy, however, refused to elaborate on whether or not the U.S. government has made any progress in its discussions with Pyongyang to win Bae's release.

King said the U.S. has watched the progress of recent rounds of inter-Korean discussions "with interests."

After months of simmering tensions on the Korean Peninsula triggered by the North's third nuclear test in February and bellicose threats against South Korea and the U.S., Pyongyang shifted to a charm offensive around June.

As a sign of thawing inter-Korean relations, Seoul and Pyongyang recently agreed to restart the shuttered joint industrial park in the North's border town of Kaesong. The two countries also arranged reunions for families separated by the 1950-53 Korean War.

“We'd like to see North Korea make progress in the areas of human rights," King said as a condition for his possible trip to the North, adding that he is "not planning (to visit the North) right now."

Calling his meetings with South Korean officials "very productive," he vowed to maintain a close and cooperative relationship with Seoul to determine how to deal with the communist nation.    

He has been in Seoul since last week for his six-day visit.

King met with senior officials of the presidential office, the foreign ministry and the unification ministry.

On Tuesday, he is scheduled to deliver a keynote speech at a forum hosted by the state-run Korea Institute for National Unification on the North's human rights conditions.

Appointed in 2004, King is in charge of dealing with humanitarian assistance from the U.S. to the North, support for North Korean defectors settling in the country and other legal efforts to curb the communist country's human right violations. (Yonhap News)