The U.S. military is closely watching a North Korean ship that may be carrying banned cargo related to its nuclear and missile programs, a U.S. defense official said Thursday.

It was the first vessel to be tracked under new U.N. sanctions imposed on Pyongyang last week after it carried out an underground nuclear test May 25.

"There is a particular ship that we are closely monitoring," a defense official, who spoke on condition of anonymity, said.

The U.S. military had been tracking the ship, the Kang Nam, for "several days now," said the official.

He said the military had "a variety of ways keep an eye" on the vessel, without specifying U.S. Navy ships at sea, submarines and satellite surveillance.

The U.N. Security Council resolution adopted a week ago bans shipments of arms and nuclear and missile technology to and from North Korea, allowing for inspections of cargo.

Under compromise language favored by China and Russia, the U.N. resolution rules out the use of military force to back up the inspections.

The U.S. military has long kept a close watch on ships heading in and out of North Korea and the new U.N. resolution means "we have newfound authorities and responsibilities," the official said.

The U.N. sanctions allow for the U.S. Navy and its allies to ask to inspect North Korean vessels and ships flagged from other countries suspected of carrying banned cargo.

"We intend to vigorously enforce the United Nations Security Council Resolution 1874 to include options, to include, certainly, hail and query," Admiral Mike Mullen, the top U.S. military officer, told a news conference earlier Thursday.

If the ship refuses the search, then the vessel would be directed to a nearby port, Mullen said.

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