North Korea's state media, which in the past reported on the detention of U.S. nationals, have also not commented on the incident so far.
"I don't think anyone who was sane would want to go to North Korea, so I assumed it was some kind of stunt."
State Department spokesman Miller said Sweden has been engaged as it acts as a diplomatic channel for Washington which remains technically at war with North Korea.
North Korea has previously detained Americans who entered the country and put them on trial but eventually released them, often following high-level diplomatic intervention.
A former North Korean diplomat who defected to South Korea said King may be used as a propaganda tool, but it was not clear how long North Korea would want to exploit his presence.
Persons:
Kim Hong, Ji, Travis T, Matthew Miller, King, Sarah Leslie, Miller, Karine Jean, Pierre, Biden, Otto Warmbier, hyang Choi, Phil Smith, Steve Holland, Jack Kim, Ed Davies, Lincoln
Organizations:
REUTERS, State Department, Pentagon, Korean People's Army, North, Joint Security Area, New Zealand, Reuters, . State, U.S, United Nations Command, Thomson
Locations:
Panmunjom, South Korea, U.S, Sweden, North Korea, SEOUL, WASHINGTON, Washington, Pyongyang, North, United States, New, Korea, Korean, Seoul