The US defence secretary says steps are being taken to protect against a possible North Korean missile launch across the Pacific.

Robert Gates said on Thursday that the US had positioned more missile defences around the state of Hawaii.

The news comes as US officials also said they were monitoring a North Korean ship for the first time under new United Nations sanctions imposed after its nuclear test and missile launches last month.

Gates told reporters at the Pentagon on Thursday that he had sent the military's ground-based mobile missile system to Hawaii, and positioned a radar system nearby following "some concerns" of a new test.

"Well, we are obviously watching the situation in the North with respect to missile launches very closely and we do have some concerns if they were to launch a missile to the west in the direction of Hawaii," he said.

US and Japanese sources have said that North Korea could fire its most advanced ballistic missile towards Hawaii around the July 4 Independence day holiday in the US.

Tensions on the Korean peninsula have been rising after the North conducted a nuclear test and several missile tests in May.

It has also recently warned ships to stay away from waters off its eastern city of Wonsan for the remainder of June, according to a Japan coast guard spokesman.

Ship being tracked

Meanwhile US officials said a North Korean-based vessel called the Kang Nam was being monitored as "a subject of interest" after leaving a North Korean port on Wednesday.

Officials say they think the cargo vessel is carrying weapons, with media reports saying the ship appeared to be heading for Singapore.

The ship, which may be carrying illicit weapons, was in the Pacific Ocean off the coast of China on Thursday, US officials told the Associated Press news agency.

The UN Security Council earlier this month voted in favour of imposing further sanctions following the May nuclear test, including banning weapons exports from North Korea and most arms imports into the country.

It also authorised UN member states to inspect North Korean sea, air and land cargo, allowing them to seize and destroy shipped goods that violate the sanctions.

North Korea responded to the latest UN sanctions by announcing it would start a uranium enrichment programme, which experts said could give it a second route to an atomic bomb, and that it would weaponise all its plutonium.

Analysts say the North's recent moves are aimed at rallying support domestically for Kim Jong-il, its 67-year-old leader, who appears to be laying the foundation for his youngest son to take over.