By Elaine Lies

TOKYO (Reuters) - A U.S. envoy was set to leave Japan on Friday to secure the release of an imprisoned and ailing American missionary in North Korea, a move that could signal the start of a gradual thaw in relations between Washington and Pyongyang.

Robert King, special envoy for North Korean human rights issues, is scheduled to return from Pyongyang on Saturday after a one-day trip, an official at the U.S. Embassy in Tokyo said. The official was unable to specify exact times.

The State Department has termed the trip a "humanitarian mission" and played down any connection between Bae's release and the North's sanctioned nuclear weapons program, although the planned release follows a pattern of previous periods of tension and thaws.

King secured the release of another Korean-American missionary, Jung Young Su, in 2011 as part of a trip to assess North Korean pleas for food aid.

Relations between Washington and Pyongyang have been in deep freeze since the collapse of a food deal in early 2012, when North Korea broke its promise to end its long-range rocket launches and prevented nuclear inspectors from examining its nuclear stockpiles and production.

Bae, 45, was sentenced to 15 years hard labor for attempting to overthrow the North Korean state by spreading anti-government propaganda, according to North Korean media. His health has deteriorated since he was jailed and he has diabetes.

North Korean state media said Bae started his plot to "topple" the country's government in 2006, a date that coincides with his own testimony about his arrival in China.

It accused him of infiltrating 250 students into the country, spreading "false propaganda" and of bribing North Korean citizens in a bid to bring down the government.

Bae lived in a Chinese town that borders North Korea and worked for a tour company while undertaking missionary work inside North Korea.

North Korea says it practices religious freedom but religious expression is in effect tightly controlled in a state that acknowledges total loyalty to the Kim dynasty that has ruled for three generations. North Korea features at the bottom of most independent surveys of freedom.

In online postings of one of his speeches on his missionary work, Bae described himself and a party he took to North Korea as "warriors for Christ" and told of holding a prayer meeting on a beach.

The postings have since been removed, as have all traces of Bae's involvement with a tour company operating out of China.

In a videotaped sermon, also removed from the Internet, Bae talked of bringing 300 people to a coastal town in North Korea to emulate the biblical destruction of the walls of Jericho.

Bae's family has acknowledged his deeply held religious beliefs but have suggested that his sympathy for North Korean orphans may have been behind his arrest.

(Writing by David Chance; Editing by Paul Tait)