If so, the shipment would pose the first test of a Security Council resolution October 14 banning North Korean exports of materials related to nuclear, chemical or biological weapons or the means to deliver them.

The resolution also banned imports and exports of an array of conventional weapon systems and called on member states to act cooperatively to prevent trafficking in banned materials by inspecting suspicious cargos.

US officials provided few details about the North Korean shipment, citing the need to protect intelligence sources and methods.

The defense official would not say where the ship was bound but he said that was known. Officials would not say when the ship left port in North Korea.

The US television program CBS News, which first reported on the ship, said US intelligence believed it was carrying banned military equipment.

A US official, who also spoke on condition of anonymity, suggested that the ship was known to US intelligence because of its history.

But the defense official said the ship was being watched because of its cargo.

"We believe it is carrying stuff that it shouldn't be carrying," he said.

While not confirming that the US has targetted a specific vessel, State Department spokesman Tom Casey aid the US is actively tracking transport around the globe to monitor weapons proliferation.

"We have been very active in terms of using our intelligence resources, using other resources available to us, to try and determine where there might be ... ships or aircraft or other kinds of transport of materials, either associated with weapons of mass destruction or ballistic missile programs," Casey said.

Separately, a State Department official said it would not be surprising that the US was tracking a shipment out of North Korea.

"I certainly think there is close scrutiny being paid to anything coming out of North Korea at this point," the official said.

If a lot of time and energy were being spent on tracking something, "there would have to be an assumption that there is something going on and it would be prohibited material," he said.

The ship surveillance comes amid intense diplomatic activity set off by North Korea's first nuclear test on October 9 and fears that Pyongyang may follow up a with a second test.

Japanese Foreign Minister Taro Aso said Japan had received information that North Korea would not conduct a second test, echoing a South Korean news agency report that China had been informed there would be no second test.

Chosun Ilbo, a South Korean newspaper, said Kim Jong-il had expressed regret over the first test in a meeting with a Chinese envoy and indicated a willingness to return to six-party talks if the United States eased pressure on it.

US Secretary of State Condoleezza Rice, meanwhile, met in Beijing with Chinese leaders.

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