Still, the statement comes at a time when American officials were already expressing renewed concerns about the North’s intentions. Intelligence officials have concluded that the recent long-range rocket test was successful, and reached as far as the Philippines before propelling a washing-machine-size satellite into orbit. It later tumbled, but the launching suggested a capability to toss a warhead much farther than before.

American intelligence officials said recently that, at best, the North’s missiles could hit Hawaii, and that it would be at least three years, maybe more, before that range could be extended to the continental United States.

American intelligence officials have also become concerned that the latest rocket test indicated that the country’s new leader might have decided that confrontation with the West could prove a more successful strategy to retaining power than a new attempt at difficult economic reforms.

There had been hopes that Mr. Kim — who is reported to have made modest economic changes and is portrayed as more affable than his father — might be willing to compromise with the West for economic aid. Thursday’s threat was the latest suggestion that he was more likely to follow the pattern that his father, Kim Jong-il, established when he ran the country: a cycle of a rocket launching, United Nations condemnation and nuclear testing.

“It’s a major test for Kim Jong-un,” said Koh Yu-hwan, a North Korea specialist at Dongguk University in Seoul. “Unlike the rocket launching in December, which the North has said was conducted because it was his father’s dying wish, a nuclear test will be Kim Jong-un’s decision, one for which he will be held responsible.”

The North appears to be making preparations for a possible nuclear test at the Punggye test site in northeastern North Korea, near the Chinese border. Defense Secretary Leon E. Panetta told reporters on Thursday that the North Koreans “have the capability, frankly, to conduct these tests in a way that makes it very difficult to determine whether or not they are doing it.”