WASHINGTON/SEATTLE (REUTERS) - The United States will send its North Korean human rights envoy to Pyongyang this week to seek the release of an imprisoned and ailing US Christian missionary, the State Department said on Tuesday.

Robert King, special envoy for North Korean human rights issues, will travel to Pyongyang on Friday at the invitation of the government on a "humanitarian mission focused on securing the release of US citizen Kenneth Bae," the department said in a statement.

King will ask North Korea to "pardon Mr. Bae and grant him special amnesty on humanitarian grounds so that he can be reunited with his family and seek medical treatment," it added.

The White House urged North Korea on Tuesday to grant Bae, who suffers diabetes among other conditions, clemency and allow him to return to the United States.

Asked about his mission, King told reporters in Tokyo on Wednesday that Bae had health problems.

"We're hopeful that we'll be able to make progress on that," he said.

King was visiting Tokyo ahead of his Pyongyang trip to meet with senior Japanese government officials and lawmakers including Keiji Furuya, minister in charge of issues related to the abductions of Japanese citizens by North Korea.

The release of Bae, which would remove an irritant in US relations with North Korea, also would fit into a recent pattern of conciliatory steps by Pyongyang toward its neighbours.

In April, Bae was sentenced to 15 years of hard labor after North Korea's Supreme Court convicted him of state subversion.

The court said Bae, 45, used his tourism business to form groups aimed at overthrowing the government.

Bae was detained in November as he led a tour group through the northern region of the country. His sentencing came amid acrimonious relations between Pyongyang and Washington over the reclusive state's nuclear program.

Bae's sister, Terri Chung, told Reuters on Aug. 10 that her brother had been held at a prison for foreigners and put to work plowing and planting fields.

He was transferred to a state hospital because he suffered from a range of health problems, including an enlarged heart, chronic diabetes as well as back and leg pain, she said.

North Korea has used the release of high-profile American prisoners as a means of garnering prestige or recognition, rather than economic gain, by portraying visiting dignitaries as paying homage to the country and its leader.

That pattern has complicated the response from US lawmakers and the State Department, which has called for Bae's immediate release on "humanitarian grounds," but had resisted sending high-profile envoys to negotiate.

Reports last month that former US President Jimmy Carter was set to visit North Korea to negotiate for Bae were denied.

In an Internet petition urging President Barack Obama to secure "special amnesty" for his father, Jonathan Bae wrote: "We have been distraught by my father's deteriorating health and his sentence of hard labor.

However, we have not given up hope because we have faith in our government to represent and protect American citizens here and abroad."

King has experience negotiating in the reclusive state, which has sought to use imprisoned foreigners as high-profile bargaining chips.

Jun Young Su, an American citizen detained in North Korea on unspecified charges for six months, was released during a May 2011 visit by King to assess its pleas for food aid.

Jun was arrested and admitted committing a crime "against the state" following an investigation, North Korea's official KCNA news agency reported.

Media reports said the California businessman had been doing missionary work in North Korea, which said he would be released on "humanitarian grounds."

Two American journalists arrested in 2009 by North Korea were held until former President Bill Clinton traveled there to negotiate their release.

"Many people will read too much into (Bae's release). What we've seen over the decades is that North Korea alternates between belligerent threats and even aggressive actions and charm offensives," said Bruce Klingner, a Senior Research Fellow for Northeast Asia at The Heritage Foundation.

In dispatching King, rather than a more prominent diplomat or former US president, the State Department weighed the well-being of a citizen against gestures that might undermine its policy toward a bellicose state, Klingner said.

Earlier this year, North Korea threatened to stage nuclear attacks on the United States, among other military actions.

Rick Larsen, a US congressman from Washington state, where Bae's family lives, has called for Bae's release.

"Kenneth's family has waited in anguish and uncertainty, but has never wavered in their tireless advocacy on his behalf,"Larsen said in a statement.