WASHINGTON — North Korea has rescinded its invitation for a senior US envoy to travel to Pyongyang to seek the release of a detained American, the State Department said Friday. Bob King, the US special envoy for North Korean human rights, was due to travel from Tokyo to Pyongyang on Friday to request a pardon and amnesty for Kenneth Bae and return the next day. Bae was sentenced in April to 15 years of hard labor by the authoritarian state, accused of subversion. Department spokeswoman Marie Harf said the US is “surprised and disappointed by North Korea's decision” and remains gravely concerned about Bae's health. King's visit would have been the first public trip to North Korea by an administration official in more than two years and could have provided an opening for an improvement in relations. US-North Korean relations are already severely strained by concerns over Pyongyang's nuclear program. King intends to return to Washington from Tokyo on Saturday. “We have sought clarification from the DPRK about its decision and have made every effort so that Ambassador King's trip could continue as planned or take place at a later date,” Harf said in a statement, referring to the country's formal title of the Democratic People's Republic of Korea. “We remain gravely concerned about Mr. Bae's health and we continue to urge the DPRK authorities to grant Mr. Bae special amnesty and immediate release on humanitarian grounds.” Bae, a 45-year-old tour operator and Christian missionary, was arrested last November and accused of committing “hostile acts” against North Korea. He suffers multiple health problems and was recently hospitalized. On Wednesday in Tokyo, King had cautioned that Washington had received no guarantees from Pyongyang that Bae would be freed. North Korea has previously used detained Americans as bargaining chips in its standoff with the US 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. 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. Meanwhile, Kim Jong-un appears to have replaced his hawkish, aging army chief in what analysts suggested Friday was a further bid to tighten control over the military. Kim Kyok-sik, 75, is believed to have been replaced as chief of the army's general staff by Ri Yong-Gil who, until now, has headed the army's general staff operations department. The North on Sunday held a meeting of the powerful central military commission where personnel changes were made, state media reported. — Agencies