But US officials, speaking privately, would not rule out direct contacts at some point to advance efforts to rein in North Korea's development of atomic weapons.

"The United States is not going to engage in bilateral negotiations with the government of North Korea," White House spokesman Tony Snow told reporters.

"We're going to continue to do it through the appropriate forum," he said referring to six-way talks among the United States, the two Koreas, China, Russia and Japan.

Snow was responding to a North Korean invitation to Christopher Hill, the head of the US delegation to the multilateral talks, to go to Pyongyang in an apparent bid to jump start the stalled negotiations.

"The United States sticks by its position, which is North Korea has to return to the six-party talks. It also has to go ahead and fulfill the obligations in the September agreement," he said.

North Korea's foreign ministry spokesman said on Thursday that Hill, a US assistant secretary of state, would be welcome in Pyongyang if Washington sincerely wanted to uphold a joint statement agreed last September at the multilateral talks.

"If the US has a true political intention to implement the joint statement we kindly invite once again the head of the US side's delegation to the talks to visit Pyongyang and directly explain it to us," the North Korean spokesman told the official Korean Central News Agency (KCNA).

In September, North Korea agreed to dismantle its nuclear program in return for security guarantees, diplomatic concessions and energy aid.

But two months later, it launched a boycott of the talks after Washington imposed financial sanctions aimed at curbing Pyongyang's alleged US dollar counterfeiting and money laundering activities.

The State Department made it clear Thursday that the United States was already involved in direct talks with North Korea within the six-party negotiation process.

"Well, I don't think the issue here is really direct talks between the United States and North Korea. We have direct talks with the North Koreans in the context of the six-party talks," acting State Department spokesman Tom Casey told reporters.

A senior US administration official told AFP that North Korea had not formally invited Hill.

"No-one has received any communication from the North Koreans, all we've seen is some press reports. North Korea has not extended, to us, any invitation," the official said on condition he not be named.

A State Department official, also speaking on condition of anonymity, stopped short of categorically rejecting the North Korean invitation.

Hill has said previously that he would go to North Korea if it was determined "by the collective powers that be" that it would be a useful thing to do in order to advance the six-party talks, the official said.

"So I think people need to take a look at this and figure out exactly what it is and see if there is anything there that would warrant determining that 'yes, now is the time that this would be a useful gesture,'" the official said.

"I didn't want to rule it out out of hand but I didn't want to make it sound like there is anyone saying 'this is a great idea, let's do it tomorrow'," said the official, noting that Hill was currently "doing some personal travel."

North Korea declared last year that it had nuclear weapons, deepening a standoff which began when the United States accused the communist state in 2002 of secretly enriching uranium, a process that could lead to manufacture of nuclear bombs.

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