North Korea has rescinded an invitation for a U.S. envoy to visit North Korea and try to secure the release of a jailed American Christian, CNN reports. Ambassador Robert King, President Barack Obama's special envoy for North Korean human rights issues, was expected to fly to the Asian nation on Friday to try to win the freedom of Kenneth Bae, an American citizen imprisoned there for carrying out "serious crimes" against Kim Jong Un's regime. North Korean authorities detained Bae, a tour operator and Christian missionary, last November and sentenced him to 15 years of hard labor earlier this year. They said he had planned an operation to bring down the government through religious activities. Bae's sister, Terri Chung, said earlier this month that his health has severely deteriorated during his imprisonment and that he has been transferred to a hospital. He has spent more time in North Korean custody than any other American.