At a six-nation meeting in China that included the United States, North Korean Deputy Foreign Minister Kim Yong Il also said, according to the U.S. official, that his country has the means to deliver nuclear weapons, an apparent reference to its highly-developed missile program.
The State Department declined comment on the deliberations in Beijing except to reiterate that the U.S. goal at the conference is to focus on "the complete, verifiable and irreversible elimination" of North Korea's nuclear weapons program.
CBS National Security Correspondent David Martin reports
the North Koreans have a long history of delivering dire threats. But if they were to make good on this one, it would turn what is currently a diplomatic impasse into a full blown crisis.
The CIA believes North Kkorea already has enough nuclear material to make one or two weapons, and missiles that could deliver them as far as Alaska and the Hawaiian islands, reports Martin.
U.S. officials are not certain whether the North Korean dictator Kim Jong II is determined to build nuclear weapons or is simply trying to extract the best deal possible for dismantling his program.
But to formally declare and then to test a nuclear weapon would remove all doubt and run smack into President Bush's vow that he will not tolerate nuclear weapons in North Korea, reports Martin.
Wie Sung-rak, director-general of the South Korean Foreign Ministry's North American Affairs Bureau, injected a positive note, saying in Beijing that another round of talks probably will be held after the current round ends on Friday. Efforts to confirm Wie's statement with U.S. officials were unsuccessful.
North Korea had confirmed privately to U.S. officials last April during talks in China that it possessed nuclear weapons but Kim's statement Thursday is believed to have been its such acknowledgment in a formal setting.
Present for Kim's presentation at a guest house in western Beijing were Assistant Secretary of State James A. Kelly and representatives from China, South Korea, Japan and Russia, in addition to North Korea.
U.S. intelligence has not detected overt signs that North Korea is preparing to conduct a nuclear weapons test, said one U.S. defense official, speaking on condition of anonymity. But such a test would presumably be underground, so preparatory work would be difficult to detect, the official said.
There was speculation here that North Korea could carry out a nuclear test on Sept. 9, the anniversary of the formation of the Democratic People's Republic of Korea, as the country is known officially.
With the exception of North Korea, all governments represented in Beijing had expressed varying degrees of opposition to the communist country's nuclear programs.
The administration believed that a broad international front — including North Korea's neighbors — in support of a denuclearized Korean peninsula would induce Pyongyang to retreat from its nuclear ambitions.
Officials also hoped that North Korea might show flexibility given U.S. promises to offer security guarantees to Pyongyang as well as measures to assist the country's stricken economy.
In addition, the administration found encouragement in signs that China was increasingly opposed to North Korea's program. As a major donor of food and energy aid to North Korea, China has been thought by U.S. officials to be in position to play a decisive role in restraining Pyongyang.
But the North Korean rejection of the U.S. nuclear disarmament proposal appeared to be complete. According to the official, Kim said there was no evidence of any U.S. intention to abandon its policy of hostility toward North Korea. He also rejected U.S. suggestions that North Korea open up its nuclear facilities to international inspection.
The administration official, asking not to be identified, said China's delegate appeared visibly angry over Kim's statement but responded in a moderate tone.
The official said that when Russia and Japan attempted to point out some positive elements of the U.S. presentation, the North Korean delegate attacked them by name and said they were lying at the instruction of the United States.
North Korea has insisted for almost a year that Washington and Pyongyang negotiate a non-aggression pact. But the administration has shown no interest. American promises of no hostile intent toward North Korea apparently have not satisfied Pyongyang.
It is widely suspected that President Bush generated anxiety in Pyongyang by including North Korea, along with Iran and pre-war Iraq, as a member of an "axis of evil" in January 2002.
Also, the White House, in a September 2002 global strategy report, threatened to stop North Korea and other hostile nations before they are able to attack the United States with weapons of mass destruction.
Although there is no clear evidence of a connection, North Korea in the months after the report was issued took a series of steps that caused alarm in Washington, Northeast Asia and beyond.
They included Pyongyang's withdrawal from the Nuclear Non-Proliferation Treaty, cancellation of an eight-year freeze on its plutonium-production program and assertions that reprocessing had begun on spent nuclear fuel rods — an essential step toward production of nuclear weapons.
The administration began exerting pressure on North Korea starting last fall after receiving information that the country had launched a uranium-based nuclear weapons program.
The administration said North Korea confirmed the existence of such a program to Kelly during a meeting in Pyongyang last fall. On Thursday, however, Kim, according to the U.S. official, said his country has no such program.