The White House and State Department insisted that the United States and its partners in the process -- China, Japan, Russia, and South Korea -- were "unified" in seeking an end to Pyongyang's nuclear weapons programs.

"There is a great deal of unanimity," White House spokesman Tony Fratto said one day after the highly unusual broadside from President George W. Bush's special North Korea envoy for human rights, Jay Lefkowitz.

"We do believe that the five parties of the six-party talks, who are encouraging North Korea to relinquish their nuclear program, stand together and are unified in that effort," said Fratto.

"At the end of the day, the only voice that matters is that of the president of the United States, and this is his policy," State Department spokesman Sean McCormack said of the multilateral approach.

McCormack praised Lefkowitz's personal qualities but stressed: "He is not, however, somebody who speaks authoritatively about the six-party talks. His comments certainly don't represent the views of the administration."

McCormack said that chief US nuclear envoy Christopher Hill -- not Lefkowitz, who became special envoy in August 2005 -- was qualified to discuss the state of talks meant to disarm North Korea.

Later, the State Department formally denied the envoy's claim that the strategy for dealing with the Stalinist state, which tested a nuclear bomb in October 2006, was under review.

Washington has expressed growing frustration that North Korea missed a key December 31 deadline to disable its main nuclear facilities and fully disclose its atomic programs in return for economic aid under a February 2007 deal.

The delay is believed to be over North Korea's reported refusal to provide information on US evidence that the isolated regime maintained a secretive uranium enrichment program alongside its plutonium powered nuclear plant.

Lefkowitz charged in a speech late Thursday that North Korea used its nuclear arsenal to "extort" foreign aid, was "not serious" about disarming, and would likely not give up its weapons before Bush's term ends in January 2009.

He also called for a "new approach" in disarmament talks -- "perhaps even bilaterally" -- with North Korea that would permanently link human rights as part of the engagement policy and a critical condition for any normalization of diplomatic relations.

In addition, he said that China and South Korea -- the two nations with the most leverage over the North Korean regime -- were "unwilling to apply significant pressure on Pyongyang" to abandon its nuclear weapons arsenal.

It was unclear whether Lefkowitz's remarks were a warning shot across North Korea's bow in a bid to speed its declaration, or a sign that critics of the engagement strategy were asserting themselves after the unmet deadline.

One former senior US official who has criticized the six-party process, John Bolton, said that Lefkowitz's sharp words showed that a split inside the US government on how to proceed on North Korea was "definitely still alive."

"And, I think, alive in the president's own mind," said Bolton, who served Bush as under secretary for arms control and international security and US ambassador to the United Nations.

"I don't think he likes the situation he is in on North Korea. And I still think there is a chance that North Korea's non-performance can yet lead to the president to repudiate the February 2007 deal," he said.

Bolton said that North Korea needed to be "much higher" on Washington's list of priorities for the US-China relationship, adding "we need China to take a stronger role.

Pyongyang on Wednesday accused US hardliners of trying to wreck the nuclear deal, saying the issue would never be resolved by pressure tactics.

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