US officials said they could not be certain what the North Koreans were doing, but the activity could be preparations for a second nuclear blast, NBC and ABC said.
The news prompted immediate concern from Japan, China, South Korea and the United States and warnings that further measures would be taken following a UN Security Council resolution on Saturday that slapped sanctions on Pyongyang.
Asian stock markets twitched nervously, with both Japan and South Korea slipping into the red.
"(We have) to really make it clear that North Korea will pay a very, very high price for this type of reckless behaviour," said US Assistant Secretary of State Christopher Hill as he arrived for crisis talks in the South Korean capital on Tuesday.
He said he had heard the latest reports, but -- like officials from South Korea and Japan -- had no specific or detailed information.
"I think North Korea has put itself on a course to much deeper isolation from the international community and ultimately to a very, very bleak future," he said.
Japanese media reported that China, North Korea's closest ally and the least enthusiastic about sanctions, was halting some remittances to Pyongyang through its state bank.
The daily Asahi Shimbun quoted a Bank of China official as saying the move was related to the UN sanctions adopted unanimously by the Security Council on Saturday in response to Pyongyang's Oct. 9 nuclear test.
But it also appeared to be part of wider steps by Chinese banks to restrict North Korean activities that started earlier this year after the US Treasury imposed financial restrictions on the grounds North Korea counterfeited money and was involved in illegal drug trafficking.
Sources were quoted by South Korean media as saying it did not look as if the Chinese government was ordering banks to crack down but rather that banks have started doing so on their own.
STATE COLLAPSE
China's worry that tough action could provoke a collapse of the impoverished and highly militarised state has prompted concern it would not enforce the sanctions.
But US Secretary of State Condoleezza Rice, on the eve of a trip to the region, dismissed the scepticism.
"I don't think they would have voted for a resolution that they did not intend to carry through on."
Rice was due in Tokyo, Seoul, Beijing and Moscow this week to try to cement the unified approach to North Korea and restart the six-party talks, a forum for discussions aimed at persuading North Korea to give up its nuclear weapons ambitions.
US Under Secretary of State Nicholas Burns said China was already taking action to check goods crossing into North Korea. "The Chinese now are beginning to stop trucks at the 800-mile (1,400 km) border and inspect all of them," he said on CNN.
Japan has announced it was extending already harsh restrictions and Australia said it was prohibiting North Korean ships from entering Australian ports. It was unclear how many North Korean ships visited Australia to start with, however.
Professor Nam Sung-wook of Korea University, an expert on North Korea, said the practical effect of sanctions was questionable.
"They (countries supporting sanctions) are in bed together but they're all dreaming different dreams," he said.
The US government has now confirmed that the Oct. 9 explosion, which prompted world-wide condemnation and the harsh sanctions regime, was a nuclear explosion as Pyongyang claimed.
"Analysis of air samples collected on October 11, 2006, detected radioactive debris which confirms that North Korea conducted an underground nuclear explosion," the director of national intelligence said in a statement.
The New York Times said in its early Tuesday edition that the explosion was most likely not fuelled by uranium, but rather by plutonium harvested from its small, mothballed nuclear reactor.
The report, which quoted unnamed administration and intelligence officials, suggests fears North Korea had developed a uranium programme based on equipment and know-how from Pakistan were unfounded.
North Korea's ambassador has told the Security Council that if pressure on his country increased, Pyongyang would "continue to take physical countermeasures considering it as a declaration of war".
The reclusive state blasted Japan, the United States and South Korea in various commentaries carried by its KCNA (Korean Central News Agency) mouthpiece on Tuesday.
"Their reckless behaviour is a challenge to the trend of the times," it said in one article, referring to Washington and Seoul.

Copyright Reuters, 2006