The North Korean claims were reportedly made during three days of nuclear talks in Beijing that ended Friday.
During the talks, Pyongyang said it presented a new proposal to resolve the dispute, but it was ignored. U.S. officials have said they are seeking the "verifiable and irreversible" elimination of the North's nuclear weapons program.
Despite the apparent impasse, both sides agreed to meet again, according to China's Foreign Ministry, which hosted the meeting.
North Korean delegate Ri Gun told U.S. Assistant Secretary of State James Kelly that North Korea had reprocessed all 8,000 spent nuclear fuel rods in its possession, a senior U.S. official in Washington said on condition of anonymity.
The official said Ri made the comments about the fuel rods at a plenary session while the other comments on its nuclear activities were made at a social gathering Wednesday after formal discussions.
If the North's alleged claims of reprocessing are true, that would put it much closer to building six to eight additional weapons beyond the one or two it is believed to have.
A U.S. official said Thursday night it was doubtful the North Koreans have reprocessed or made significant progress in reprocessing. Large-scale reprocessing probably would be detectable via satellite photos, the official said.
South Korean officials said Friday that they were checking the North Korean claims of having nuclear weapons. Some analysts suggested that the North was bluffing in order to pry concessions from the United States.
Ko Yoo-hwan, a North Korea expert at Seoul's Dongkuk University, said if the North "admitted that it was armed with nuclear weapons, it means it is using its last card. It also means it's going for its last, big deal with the United States."
A U.S. official said there were no indications that a nuclear test by North Korea was imminent but acknowledged that preparations for an underground test could be concealed.
The talks were the first high-level U.S.-North Korean contact since tensions over the North's nuclear ambitions spiked in October. That's when Washington claimed that the North said it had a secret nuclear program in violation of a 1994 pact. The North has disputed that it made that claim.
In a report carried by the North's KCNA news agency, a North Korean Foreign Ministry spokesman said Ri "set out a new proposal for the settlement of the nuclear issue." The report did not elaborate on the contents of the offer.
In a possible reference to North Korea's demand for a nonaggression treaty with Washington, the spokesman said Kelly "persistently avoided the discussion on the essential issues to be discussed between both sides."
U.S. officials have ruled out such a treaty but say some sort of written security guarantee might be considered.
North Korea had repeatedly said it believes the United States plans to invade it after the Iraq war.
Kelly departed for Seoul soon after the talks. At Beijing's airport, Kelly said only that he had "a good visit to Beijing."
Secretary of State Colin Powell said any attempts by the North Korean government to intimidate the United States would fail. President Bush said the talks give the United States "an opportunity to say to the North Koreans, 'We are not going to be threatened.'"