North Korea said Saturday that it had to cancel the planned weekend visit of a special U.S. envoy to the country because of fresh U.S. military provocations.

Robert King, special envoy on North Korean human rights issues, had planned to begin a two-day visit to the communist nation on Friday to secure the release of Kenneth Bae, a 45-year-old Korean-American man serving a prison term there.

North Korea said it had agreed to allow King's visit on humanitarian grounds but the U.S. "ruined" the atmosphere of dialogue by flying B-52H strategic bombers over the Korean Peninsula for nuclear bombing drills.

"The strategic bombers' intrusion into the air over the Korean Peninsula is the most blatant nuclear blackmail against us and a military threat to us," the North's foreign ministry said in an English-language statement carried by the country's Korean Central News Agency.

"The U.S. thus beclouded the hard-won atmosphere of humanitarian dialogue in a moment." it said.

South Korea and the U.S. ended a 12-day joint military drill, called Ulchi Freedom Guardian, on Friday. It was not immediately known whether the U.S. had flown the strategic bombers during the drills.

On Friday, Marie Harf, deputy spokeswoman for the U.S. State Department, expressed disappointment over the North's abrupt decision to cancel King's trip, saying that Pyongyang did not provide a clear reason for its decision.

North Korea claimed in the statement that it had clearly explained to the U.S. why it had canceled King's trip but U.S. officials are spreading "misinformation" about its intentions.

"It is something surprising that the U.S. is making irrelevant remarks that it was surprised by our action," the North's statement said.

The Korean-American man was detained during a trip to North Korea in November on charges of committing hostile acts against the regime. He was later sentenced to 15 years of hard labor.

King's trip had been expected to provide a rare opportunity for North Korea and the U.S. to open dialogue for improved relations.

The two countries have no formal diplomatic relations.

About 28,500 U.S. soldiers are stationed in South Korea, a legacy of the 1950-53 Korean War. (Yonhap news)