Robert King had been scheduled to visit Pyongyang to ask the government to grant amnesty to Kenneth Bae.
Bae, a U.S. citizen of Korean descent, was arrested in North Korea last year while attempting a Christian mission and sentenced to 15 years hard labor in April.
"We have sought clarification from the DPRK (Democratic People's Republic of Korea) 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," the department said.
King planned to return to Washington from Tokyo Saturday, the statement said.
The department said U.S. officials are "gravely concerned" about Bae's health.
Bae was transferred to a Pyongyang hospital from a prison camp this month, and his family in Edmonds, Wash., has said they are worried about his health, The New York Times reported.
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