SEOUL, South Korea — Washington’s special envoy on North Korean human rights issues will visit that country on Friday to try to secure the release of an American imprisoned there on charges of plotting to harm the socialist country through his Christian missionary activities, the State Department said on Tuesday.

The envoy, Robert King, will travel at the invitation of the North Korean government, the State Department said in a news release.

Although the statement characterized Mr. King’s trip as “a humanitarian mission” focused on helping to free the Korean-American, Kenneth Bae, 45, whose health is reportedly deteriorating, analysts will monitor it closely to see if it can lead to a resumption of official dialogue between the United States and North Korea.In an interview with Chosun Sinbo, a pro-North Korean newspaper in Japan, earlier this month, Mr. Bae appealed to Washington to send a high-ranking official to Pyongyang to “apologize” for his crime and help free him.

“Ambassador King will request the D.P.R.K. pardon Mr. Bae and grant him special amnesty on humanitarian grounds so that he can be reunited with his family and seek medical treatment,” the State Department said, using the acronym of North Korea’s official name, the Democratic People’s Republic of Korea.