The Pentagon confirmed that B-52s, taking off from Andersen Air Force base in Guam, had flown over South Korea as part of annual joint exercises that Pyongyang insist are a rehearsal for provocation.
"We cannot tolerate the US carrying out nuclear strike drills, setting us as targets, and advertising them as strong warning messages," a spokesman for the North's supreme army command said.
"The US should not forget that the Anderson base ... as well as naval bases at Japan's main island and Okinawa, are all within the range of our precision target assets," he said in a statement carried by the Korean Central News Agency.
Military tensions on the Korean peninsula are at their highest level for years, with North Korea -- angered by UN sanctions imposed after its nuclear test last month -- threatening a second Korean War backed by nuclear weapons.
B-52s have taken part in South Korea-US joint exercises before, but the Pentagon said it had publicised their use this time to underline US commitment to defending its South Korean ally.
"As the US has started naked nuclear blackmail, we will move on to corresponding military actions," the North's army spokesman warned without elaborating.