Siegfried Hecker, a former director of the Los Alamos, N.M., nuclear research laboratory, said the North Koreans "most likely" have the ability at the Yongbyon nuclear site to make plutonium metal.

But, he said, he saw no convincing evidence that the North Koreans could use that metal to build a nuclear device. And even if they had that capability, he said he saw no proof the North Koreans could convert such a device into a nuclear weapon.

Hecker added that he was also unable to substantiate a North Korean claim that 8,000 fuel rods were reprocessed last year to extract plutonium metal - an essential step in nuclear weapons development.

The nuclear scientist went to North Korea with several American colleagues on an unofficial visit two weeks ago.

In testimony before the Senate Foreign Relations Committee, Hecker said the North Koreans apparently wanted to show the delegation their main nuclear site "to verify that they had taken significant actions since December 2002 and to impress us with their nuclear capabilities."

Hecker said his hosts seemed disappointed when he reported to them that he had not seen enough to draw definitive conclusions about the facility.

The Bush administration has believed for some time that North Korea has at least one nuclear weapon. It has been worried about the possibility of North Korean attempts to sell nuclear technology to terrorist groups or rogue states.

The U.S. government neither facilitated nor discouraged the mission to North Korea. Participants have provided briefings to administration officials.

While showing interest in the group's conclusions, the administration has said its focus is on achieving nuclear disarmament in North Korea through a six-nation process that got under way last summer in Beijing.

Efforts since then to arrange a second meeting have not been successful because the parties have been unable to reach agreement on ground rules. Besides the United States and North Korea, other nations taking part are South Korea, China, Japan and Russia.

One of the most divisive issues between the United States and North Korea concerns the U.S. contention that Pyongyang is attempting to develop a uranium bomb in addition to its plutonium bomb project in Yongbyon.

The Bush administration bases its claim on an October 2002 meeting in Pyongyang in which, according to U.S. officials, North Korea acknowledged the uranium bomb project. That allegation has drawn repeated North Korean denials.

John Lewis, a Stanford University professor emeritus who organized the mission to North Korea, said Wednesday in a telephone interview that he believes the disagreement may have resulted from a problem in the translation from Korean to English.

Heading the U.S. delegation at the meeting was Assistant Secretary of State James Kelly. Vice Foreign Minister Kang Sop Ju led the North Korean delegation.

The Bush administration maintains that Kelly confronted Kang with intelligence information disclosing the uranium bomb program and that Kang surprised Kelly and his colleagues by confirming the existence of the program.

But Lewis said a North Korean transcript of the meeting quoted Kang as saying, "We are entitled to have a nuclear program."

Lewis said that, in the Korean language, there is "a small difference between to have and entitled to have."

When Kelly pressed Kang whether he was acknowledging the uranium program, he was told: "It's up to you to think about this. We will not take the trouble to interpret this for you," Lewis said.

Lewis said North Korea has offered to have technical talks with the United States to clarify the disagreement. Given the high stakes involved, Lewis said he believes it is important that such discussions take place.

Hecker, during his congressional testimony, said the North Koreans provided the visiting delegation with a transcript of the Pyongyang meeting in 2002 for delivery to the State Department.

Department spokesman Adam Ereli said Wednesday the administration stands by what it has said all along about the outcome of the meeting.

"There was no doubt in the minds of the officials who were in the meeting or in the translations that were made of the comments, and subsequently analyzed, about what was said and what was its import," Ereli said.

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