North Korea's increasingly sophisticated missile technology is a concern to Asia and the United States, the new US Pacific commander said Wednesday ahead of Pyongyang's planned rocket launch.

"We have seen over time the North Koreans pursue increasingly sophisticated ballistic missile defence technologies," Admiral Samuel Locklear, the head of the US Pacific Command based in Hawaii, said in Tokyo during his first visit to Japan in the role.

"And I understand that if they are able to achieve their capabilities over the long run that they are pursuing... it will increase the potential ranges of the missile technology that they have and they could proliferate.

"And this... will be a concern for the alliance, a concern for the region as well as a concern for the United States," he said.

He declined to say how far advanced he believed North Korea's missile technology was, but added "hopefully the North Koreans would make the decision to de-escalate... and they would not continue to pursue the missile technology which has a destabilising effect on the security of the region."

Shortly after Locklear spoke, North Korea announced it was fuelling the rocket it says will propel a satellite into orbit to collect data on forests and natural resources within its territory.

The West says it is a disguised ballistic missile test, in violation of a United Nations ban, and fears that North Korea will follow up with a third nuclear test.

Locklear, who oversees more than 300,000 service members and a fleet of aircraft and warships over an area spanning the west coast of the United States to the western border of India, said he knew little about any atomic test.

"Beyond (what) you have seen in that open press reporting, I have no further information," he said.

Locklear was nominated for the post in December after a year in which US President Barack Obama repeatedly stressed the strategic importance of the Asia-Pacific region and vowed to expand the American military's presence, announcing the deployment of up to 2,500 US Marines in Australia.

The admiral told reporters in Tokyo he was looking to strengthen existing alliances in the region.

"When you look at... the rebalancing, you should look at the totality of what's happening within the Japanese-US alliance... the cooperation, the interoperability. It goes well beyond just the issue of ballistic missile defence" that Tokyo and Washington are jointly developing, he said.

"It goes into information-sharing. It goes into cyber. It goes into all the aspects that make a good alliance better."

A US congressional advisory report released last month said China's cyber warfare capabilities would pose a danger to US military forces in the event of a conflict over Taiwan.

The report by defence contractor Northrop Grumman for the US-China Economic and Security Review Commission said China's People's Liberation Army (PLA) has placed great emphasis on what is known as "information confrontation".

The US "commitment to Japan and the region is unwavering, and we are working to ensure that our alliance is fully capable of meeting all the challenges we might face in the future," Locklear said.