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WASHINGTON (CNN) -- A preliminary analysis of air samples from North Korea shows "radioactive debris consistent with a North Korea nuclear test," according to a statement from the office of the top U.S. intelligence official.

The statement, from the office of Director of National Intelligence John Negroponte, was sent to Capitol Hill but not released publicly. CNN obtained it from a congressional source.

If confirmed, the nuclear weapons test that North Korea claimed it conducted on Monday would be the first of its kind since Pakistan's underground blast in 1998. (Watch information on first evidence of radioactivity -- 1:23)

Pyongyang's claim has renewed fears of a regional arms race and that North Korea might aid terrorists with nuclear materials or technology. (Full story)

The national intelligence office statement said the air samples were collected Wednesday, and analysis found debris that would be consistent with a nuclear test "in the vicinity of Punggye" on Monday.

"Additional analysis is ongoing and will be completed in a few days," the statement said.

The South Korean Defense Ministry told CNN that the United States has informed it that radioactivity has been detected.

The report is in contrast to information provided to CNN earlier Friday from two U.S. government officials with access to classified information. Those officials said that an initial air sampling over North Korea showed no indication of radioactive debris.

The White House said it had no confirmation that the North Koreans conducted a nuclear test.

"We've seen the various press reports," said National Security Council spokesman Fred Jones. "We still have no definitive statement on the event. The intelligence community continues to analyze the data."

The U.S. Air Force flew a WC-135 Constant Phoenix atmospheric collection aircraft on Tuesday to collect air samples from the region.

The intelligence community and the military will also continue to collect air samples in the region and use satellite information to try to collect radiological data that would confirm a nuclear test, officials said. But as time goes on, it will be increasingly difficult to achieve confirmation.

Officials emphasized earlier Friday that the data collected are preliminary and provide no conclusive evidence about the North Korean event.

It is possible there was no radiological data. That could be the case if: the North Koreans successfully sealed the site; it was such a small detonation and so deep underground there was no escape of nuclear debris; or the test was actually conventional explosives.

The U.N. Security Council has agreed to vote Saturday on whether to impose sanctions on North Korea over the purported nuclear test, according to John Bolton, the U.S. ambassador to the United Nations. (Full story)

CNN's Suzanne Malveaux, Jamie McIntyre and Barbara Starr contributed to this report.