The State Department's point man on the crisis, James Kelly, meanwhile admitted that exhaustive diplomacy aimed at convening a second round of the forum was as testing as "herding cats" as he met his South Korean and Japanese counterparts.

The three allies met after South Korea urged Washington and Pyongyang to moderate their positions to allow the talks to go ahead as hoped in Beijing this month.

But the United States said for the second time in as many days that it was prepared to show up for a meeting, blaming North Korea for stalling.

"We are ready to go. North Korea has not yet agreed to a date," said State Department deputy spokesman Adam Ereli.

"It's not that we're standing in the way. We're ready to talk."

A senior US official said that Pyongyang was attempting to induce US concessions in writing on certain steps to defuse the crisis before the talks begin.

"I'm not saying that there won't be something before the talks, but you can't negotiate the round before the round," the official said on condition of anonymity.

"You have to leave some things for the talks."

The meeting of the six countries trying to resolve the crisis -- North and South Korea, China, Japan, Russia and the United States -- was expected to take place in Beijing in the third week of December but looks set to be delayed.

Negotiations aim to persuade Pyongyang to abandon its nuclear programme and to end a crisis that erupted last year when Washington accused North Korea of breaking a 1994 nuclear freeze agreement by embarking on an enriched uranium programme.

One issue vital to hopes of resolving the nuclear crisis is "sequencing" -- the order of steps Pyongyang and Washington would both agree to take to defuse the crisis.

Under one scenario, which the Wall Street Journal reported was part of a Chinese-drafted framework, Pyongyang would renounce nuclear weapons and Washington would quickly follow up with a formal US pledge not to attack North Korea.

A broader array of other steps, including the possibility of diplomatic relations between the two Cold War foes would follow, the report said.

The senior official said such questions needed to be negotiated within the six-nation dialogue, and could not simply be agreed before it started.

"The sequence is just one idea that's out there. The Chinese are not saying that if we don't sign up for this there won't be talks but they are telling us that this is what the North Koreans want."

Kelly told reporters that Washington still hoped six-way talks could open soon.

"This is a six-party multilateral process on a very serious issue," he said.

"As one of my colleagues said today when we have six countries pursuing the details of these things, it's a little bit like herding cats. It's a very difficult process.

Kelly met Japanese envoy Mitoji Yabunaka, director general of the Foreign Ministry's Asian and Oceanian Affairs Bureau and South Korea's Deputy Foreign Minister Lee Soo-Hyuck.

Before the talks, South Korea's envoy to North Korea earlier called on both the United States and North Korea to moderate their positions to allow six-nation talks to get underway.

"North Korea should stop pressing its demands too hard. The United States is also required to ease its stance for the momentum of dialogue," Unification Minister Jeong Se-Hyun said in Seoul.

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