None of the four air samples taken from the "cloud" from North Korea's purported May 25 nuclear test prove that North Korea tested a nuclear device, a senior U.S. defense official with access to the test results tells FOX News.

All four air samples have been tested and show no signs of the radioactive isotopes that would confirm the nuclear test, believed to be North Korea's second.

The official added that the negative results in the atmospheric samples do not rule out a nuclear test. The samples simply cannot prove for certain that a nuclear device was detonated, and therefore it is impossible at this time for the Pentagon to know with any accuracy what its potential yield was.

The official told FOX News that given the North's prior test and its current statements and behavior, U.S. intelligence authorities are still operating under the assumption that a test took place last month.

The United States has told South Korea it will impose its own financial sanctions on the North apart from punishments that the U.N. has been mulling for Pyongyang's latest violation of U.N. Security Council resolutions, a news report said Friday.

The U.S. sanctions call for blacklisting foreign financial institutions that help the North launder money and conduct other dubious deals, the South Korean Chosun Ilbo newspaper reported.

U.S. Deputy Secretary of State James Steinberg briefed the South Korean president on the new sanctions at a meeting Thursday, the mass-market paper said, citing an unidentified official at the presidential office. The U.S. Embassy in Seoul could not confirm the report. Steinberg met with Chinese officials in Beijing on Friday.

Last week, Pyongyang conducted a barrage of missile launches and an underground nuclear test that violated previous U.N. Security Council sanctions. The North also appeared to be preparing for more missile tests, including one believed to be capable of reaching the U.S.

A similar U.S. measure imposed on Banco Delta Asia, a bank in the Chinese territory of Macau, in 2005 effectively led to the North being severed from the international financial system, as other institutions voluntarily severed their dealings with the bank and the North.

The measure hit the North hard, said Lee Sang-hyun, an analyst at the Sejong Institute think tank, south of Seoul. "Sanctioning North Korea has not worked except for one case, the BDA case, cutting off the financial flows," Lee said.

News reports at the time said North Korean officials had to carry around bags of cash for financial transactions. Pyongyang stayed away from nuclear disarmament talks for more than a year in retaliation.

Steinberg was in Seoul from Tuesday through Thursday to coordinate a united response to Pyongyang's belligerence. His interagency delegation included Stuart Levey, the Treasury Department's undersecretary for terrorism and financial intelligence.

Levey, who was in charge of the 2005 backlisting of the Macau bank met with South Korean Vice Finance Minister Hur Kyung-wook, and the two agreed to strengthen cooperation in the fight against money-laundering and counterfeiting, Hur's office said.

Seoul's Dong-a Ilbo newspaper had a similar report Thursday, saying Seoul and Washington have confirmed the North has kept producing high-quality fake U.S. dollar bills, known as "supernotes," and that Washington could use the counterfeiting as justification for its own sanctions.

Steinberg told South Korean President Lee Myung-bak that North Korea would be mistaken if it thinks it can uses its old tactic of making provocations and then getting what it wants through talks, Seoul's presidential office said in a statement.

The U.S. delegation left for China on Friday.

Complicating the situation, two American journalists were believed to be on trial in North Korea's top court, on allegations they entered the country illegally and engaged in "hostile acts." The North's official news agency said the proceedings were to begin Thursday but no further details were available one day later.

Meanwhile in New York, ambassadors from key nations continued to try to reach an agreement on new U.N. sanctions against North Korea for defying the Security Council and conducting a second nuclear test. Closed-door meetings have been held since May 26.

Many analysts have said that economic sanctions against the isolated North won't be effective unless China actively implements them. Pyongyang relies heavily on China for food and energy aid and imports. More than 70 percent of the North's total trade is with China.

"Without China's participation, economic sanctions will have nearly no effect," said Cho Myung-chul, an defector-turned analyst at Seoul's state-run Korea Institute for International Economic Policy.

The Associated Press contributed to this report.

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