A North Korean official has said his government no longer wants even bilateral talks with the United States over its nuclear weapons programme.
North Korea says it has nuclear weapons
Pyongyang had been demanding direct negotiations with the US for two years.
Earlier this month, North Korea confirmed it had nuclear weapons and withdrew from discussions with its neighbours and the US.
The US and Japan on Saturday urged North Korea to resume the six-party talks, expressing their concerns.
Pyongyang said earlier it would stay away from the talks for an "indefinite period" and "bolster" its nuclear arsenal.
The North Korean foreign ministry official told China's Xinhua news agency that Pyongyang no longer wanted direct talks with the US, citing what he described as Washington's persistent attempts to topple the communist regime.
"[North Korea] has no justification to take bilateral talks... on the nuclear issue of the Korean Peninsula with the United States now," the official said.
The news came as a top Chinese envoy arrived in North Korea to urge it to return to six-party talks.
China is North Korea's closest ally and has coaxed it back into talks before.
Later on Saturday, the US and Japan called on Pyongyang to return to the negotiating table.
"We share a concern about events on the Korean Peninsula," US Secretary of State Condoleezza said after talks in Washington Japanese Foreign Minister Nobutaka Machimura and Defence Agency chief Yoshinori Ono.
"The ministers and I urge North Korea to return to the six-party talks as the best way to end nuclear programs and the only way for North Korea to achieve better relations," Ms Rice said.
Diplomats trying to formulate a response to North Korea's latest nuclear claims are struggling to understand its motives in withdrawing.
Sometimes contradictory statements from officials are adding to the confusion, says the BBC's Charles Scanlon in Seoul.
Earlier, North Korea's ambassador to the United Nations said there could be a future for negotiation, but only if the US dropped its hostile policy and stopped trying to bring down the government of Kim Jong-il.
The Chinese envoy, Wang Jiarui, is head of the ruling Communist Party's international department and is expected to be in the North until Monday.
China has sponsored three previous rounds of talks and is keen to retain the diplomatic initiative concerning a country it sees as a key ally.
Diplomatic sources say Chinese officials were taken aback by North Korea's claim to have made nuclear weapons, but they have since called for patience and calm and ruled out sanctions.