North Korea recently turned over to the United States equipment found to be contaminated with traces of highly enriched uranium — HEU — apparently contradicting the country's stance that it never had such a program, FOX News has confirmed.

The equipment was described as a set of "smelted aluminum tubes" suitable for an HEU centrifuge program, a step necessary to make a nuclear weapon.

"They got some 'splainin' to do," one U.S. arms control official said when first told of the discovery about a month ago, he recalled to FOX.

However, North Korea claims the tubes were intended for use in the development of a conventional "artillery" weapon, sources told FOX News.

As part of a six-nation disarmament deal, North Korea is disabling its Yongbyon nuclear reactor and related nuclear facilities, and is obligated to provide a "complete and correct" declaration of all their nuclear programs, weapons, and materiel by Dec. 31.

American officials anxiously have been waiting to see whether, and to what extent, the North Koreans will acknowledge the existence of their HEU program, about which the United States first confronted Pyongyang in October 2002.

The Koreans initially admitted to having an HEU program, said Mike Green, a former National Security Council staff aide who was present for the October 2002 confrontation.

"It was in several sentences in a way that they were talking about it as if it existed," Green said. He summed up the initial North Korean response as: "If you want to negotiate about it, we have more — even more powerful programs. We have the right to this."

State Department spokesman Sean McCormack has confirmed as much.

During the October 2002 talks, U.S. officials "repeatedly asked, 'Are you actually saying that you have a highly enriched uranium program?' And the answer back was 'yes,' " McCormack told reporters Feb. 23.

But not long after that meeting five years ago, the North Koreans retracted their story and reverted to a stance of completely denying they ever had an HEU program.

Green told FOX News this past June he believes the data supporting the existence of the North Korean HEU program: It was not like the intelligence on Iraq's weapons of mass destruction, and that there was no dissent from any quarter of the American intelligence community about the HEU program.

The Washington Post originally reported Friday that uranium traces were found on the North Korean tubes.

Click here to read the Post report.

The North Koreans have been considerably more open about their plutonium reprocessing program, which they have used at Yongbyon to build at least one or two nuclear weapons, according to unclassified U.S. intelligence estimates.

Pyongyang conducted its first detonation of a nuclear device in October 2006. Most analysts estimate that since the North Koreans evicted U.N. nuclear inspectors from the country in late 2002 — in the wake of the confrontation with the U.S. — the Kim Jong Il regime has reprocessed enough plutonium to build up to 10 nuclear weapons.

That the North Koreans possessed aluminum tubes of concern to the U.S. and other nations has been public knowledge for some time.

The U.S. envoy to the six-party talks, Assistant Secretary of State Christopher Hill, told a Washington think tank audience on Feb. 22 that "[W]e know that they [the North Koreans] attempted to purchase some aluminum tubes from Germany. ... We have some indications that they were successful in getting some of these tubes elsewhere."

Arms control officials told FOX News Friday it is possible the contaminated equipment bore traces of HEU because it was exposed to other contaminated equipment from a source outside North Korea.

One theory is that the traces came from equipment that originated in Pakistan, where that country's chief nuclear scientist, A.Q. Khan, is now known to have secretly and illegally sold nuclear technology to countries like Iran, North Korea and Libya.

Hill told the House Foreign Affairs Committee on Feb. 28, "We know from the Pakistanis that they [the North Koreans] bought these centrifuges. There's no other purpose for a centrifuge of that kind than to produce highly enriched uranium."

Senior Bush administration officials were briefed on the discovery of the HEU traces within the last month, in highly restricted sessions held at the headquarters of the Air Force Technical Applications Center (AFTAC), located at Patrick Air Force Base in Florida.

AFTAC, according to its Web site, "performs nuclear treaty monitoring and nuclear event detection," and provides policymakers with "quality technical measurements to monitor nuclear treaty compliance and develops advanced proliferation monitoring technologies to preserve our nation's security."

James Rosen joined Fox News Channel (FNC) in 1999. He currently serves as the chief Washington correspondent and hosts the online show "The Foxhole." His latest book is "Cheney One on One: A Candid Conversation with America's Most Controversial Statesman" (Regnery, November 2, 2015).