WASHINGTON - The US military beefed up defences in Hawaii over fears of a North Korean missile launch on the Pacific island and tracked Friday a ship from the communist nation possibly carrying banned cargo.

The vessel was the first to be monitored under the UN sanctions imposed on Pyongyang last week after the Stalinist regime carried out an underground nuclear test on May 25.

US Defense Secretary Robert Gates said Thursday that Washington was watching North Korea for missile activity and that there were concerns Pyongyang might "launch a missile... in the direction of Hawaii."

He said he had approved the deployment of THAAD missile defense weaponry to Hawaii and a radar system nearby "to provide support" in case of a possible North Korean launch.

And he said that ground-based defenses in Alaska were also at the ready.

"I would just say I think we are in a good position should it become necessary to protect American territory," he said.

The Theatre High Altitude Area Defense (THAAD) weaponry, coupled with the radar system, are designed to shoot down ballistic missiles.

US and South Korean officials have said North Korea might be readying another ballistic missile test after three previous launches in 1998, 2006 and this year.

Pyongyang said its latest April 5 launch put a satellite into orbit, while the United States and its allies labeled it a disguised test of a Taepodong-2 missile theoretically capable of reaching Alaska.

But North Korea has yet to demonstrate it has the ability to build a nuclear warhead that could be fitted onto the tip of one of its ballistic missiles.

Tensions on the Korean peninsula have been running high after Pyongyang carried out its second nuclear test last month.

A US defense official confirmed that the military was monitoring a flagged North Korean ship that might be carrying nuclear or missile-related cargo in violation of new UN sanctions.

"There is a particular ship that we are closely monitoring," the defence official, who spoke on condition of anonymity, told AFP.

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

The ship could provide the first test of the UN Security Council resolution adopted a week ago that bans shipments of arms and nuclear and missile technology to and from North Korea.

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

Under compromise language favored by China and Russia, the UN resolution calls for inspections of ships but rules out the use of military force to back up the searches.

The sanctions allow for the US Navy and others 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 US military officer, told reporters at the joint news conference with Gates.

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

"The country of that port... is required to inspect the vessel and to also keep the United Nations informed, obviously, if a vessel like this would refuse to comply," he said.

Mullen would not confirm whether the military was tracking a particular North Korean vessel.

The United Nations resolution calls on member states to inspect ships if there are "reasonable grounds" that a vessel may be carrying illicit cargo.

Analysts say however that North Korea could get around the shipping measures by transporting banned cargo by air and exploiting provisions that prohibit the use of military force.

However, experts say the financial sanctions in the UN resolution could prove more effective against the isolated Stalinist state.

On June 13, the North vowed to build more nuclear bombs and start enriching uranium for a new atomic weapons program, in response to the new UN sanctions.

The Japanese daily Yomiuri Shimbun meanwhile reported that Tokyo's defence ministry believes North Korea might now be planning to launch a two-stage or three-stage Taepodong-2 missile towards either Japan's Okinawa island, Guam or Hawaii.