Initial tests of air samples taken by US planes near North Korea found no evidence of radiation, a US government intelligence official said today.
However, the official said US intelligence experts were not ready to state that the test conducted by the North Koreans earlier this week was nonnuclear or alternatively that it was a failed nuclear device.
Japan's cabinet approved tighter sanctions against North Korea today in response to its declared nuclear test this week.
Japan's sanctions include a six-month ban on all imports from the impoverished Stalinist state.
North Korea had threatened to retaliate if Japan went ahead with the measures, which are largely symbolic given the lack of strong economic ties between the two neighbours.
Nevertheless, Japan's Transport Ministry has instructed authorities and businesses involved in transportation to be fully on alert for possible terrorist acts.
The steps, adding to sanctions Tokyo had already imposed following North Korea's ballistic missile tests in July, also include prohibiting the entry of North Korean nationals except those living in Japan.
"These actions by North Korea are a grave threat to the peace and security of our country and international society and can never be permitted," Finance Minister Koji Omi said.
On Thursday, Japan's Kyodonews agency quoted a North Korean ambassador as saying Pyongyang would take "strong countermeasures" against if Japan went ahead with sanctions.
"We will take strong countermeasures. The specific contents will become clear if you keep watching. We never speak empty words," the amabassador was quoted as saying.
UN Security Council members may vote as early as Saturday on a US-drafted sanctions resolution.
China and Russia, which both hold veto power on the Security Council, were not in favour of some of the tough measures in an early draft resolution, forcing a US compromise draft. Diplomats said the fate of the new draft largely depends on Beijing.
China is anxious to avoid driving North Korea, with its 1.2 million-strong army, further into a corner, fearing it could trigger instability on the Korean peninsula, which has been divided for more than half a century.
The revised draft eliminates a blanket arms embargo but bans heavy conventional weapons such as armoured combat vehicles, attack helicopters, warships and missiles.