"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.

The conference is expected to take place in Helsinki in December under the shadow of a growing international standoff over Iran's disputed nuclear programme, which much of the West believes is a cover for a weapons drive.

"The Finnish representative was in Israel recently but it was not a secret; it was a normal meeting," Yigal Palmor told AFP of last week's talks with Finland's undersecretary of state for foreign affairs, Jaakko Laajava, details of which were first published by Haaretz newspaper.

"No decisions (on attending the conference) have been taken so far on the Israeli side but the talks are ongoing," he told AFP, without saying what was discussed.

The idea of moving toward a nuclear weapons-free Middle East won formal backing in 2010 at a meeting on the nuclear Non-Proliferation Treaty, and diplomats believe the presence of arch-foes Israel and Iran will be crucial to the success of any conference.

Israel is not a signatory to the NPT and has so far refused to commit to attending the event; Iran, which is a member of the NPT, has also refused to state a position on the proposed conference.

Laajava has been tasked with convincing both countries to attend.

Last month, Israel's UN Ambassador Ron Prosor said there could be no agreement on a weapons-free zone until there was a "comprehensive peace" in the region.

The NPT, which came into force in 1970, has been signed by 189 states. Only three countries -- India, Pakistan and Israel -- have not signed it.

Israel, which is widely believed to be the region's sole if undeclared nuclear power, believes there is a need for a more credible system for preventing the spread of nuclear weapons technology than the NPT.

"This system has failed so many times -- in Iraq, in Libya, in Syria and in Iran. We must make a more credible system but we must wait for the Arab regimes to stabilise," a senior Israeli official told AFP.

"When we're talking about proliferation, you cannot have a credible system until there is stability in the region," he said, referring to the wave of political turmoil which has engulfed much of the Arab world over the past 14 months.