South Korea's Deputy Foreign Minister Lee Soo-Hyuck will meet with top diplomats from the United States and Japan on Thursday to fine-tune preparations for the talks originally expected to take place later this month.

However, US officials warned that North Korea may be stalling over key conditions for resolving the crisis and talks could be pushed back until early next year.

Lee said in a departure statement that negotiators from China, Japan, South Korea, Russia and the United States were still hoping to meet with North Korea in the third week of December.

"It is not certain, however, whether the talks can actually be held as we plan," Lee said.

The crisis erupted after Washington said in October last year that North Korea had broken a 1994 nuclear freeze agreement by embarking on an enriched uranium programme, triggering the 14-month-old nuclear crisis.

A first round of six-way talks in Beijing in August was inconclusive and China has led efforts to persuade a reluctant North Korea to turn up for a new round.

The first indication that preparations for the talks could be in trouble came after top State Department official John Bolton warned Pyongyang Tuesday not to stall the meeting.

Asked whether Bolton was hinting there was already a hitch in the dialogue, another US official said on condition of anonymity: "He did not make any mention of the talks taking place in December."

Asked whether the meeting was more likely to happen in January or February, the official replied: "Yes."

Washington declined to spell out which issue was complicating plans to launch the talks before the end of this year.

However, North Korea recently turned down Washington's longstanding demand that it scrap its nuclear weapons verifiably and irreversibly as an initial step to dialogue.

Pyongyang says it cannot give up its nuclear weapons without a security assurance, claiming Washington plans to invade the country.

"What they are insisting on is simultaneous action. They will do something only if Washington acts too," said a foreign ministry official here.

North Korea is prepared to offer to scrap its nuclear programmes in return for an offer from Washington of a security guarantee, the official said.

Top US, South Korean and Japanese diplomats have been working for weeks on the wording of a statement promising a security assurance at the next round of six-party talks.

Lee, prior to his departure for Washington, said the three countries hoped to finalise the statement on Thursday in the US capital.

Thursday's meeting with James Kelly, US assistant secretary of state for Asian and Pacific Affairs, and Mitoji Yabunaka, director general of the Japanese Foreign Ministry's Asian and Oceanian Affairs Bureau is billed as the final opportunity to fine-tune positions before talks with North Korea.

Other potential sticking points include Washington's push for an intrusive verification regime, and Japan's determination to raise the issue of Japanese nationals abducted by North Korea.

North Korea's nuclear activities have been shut off from the outside world since the Stalinist state pulled out of the nuclear Non-Proliferation Treaty and kicked out international monitors in January this year.

Since then the Stalinst state has claimed it possesses nuclear weapons and said it had restarted a plutonium producing plant and was reprocessessing spent nuclear fuel to build more bombs.

WAR.WIRE