North Korea is confident of China's ultimate backing over the launch of a rocket that the West suspects is a disguised missile test, a Japanese defence adviser has warned.

Hiroyasu Akutsu, professor at the National Institute for Defence Studies (NIDS), said the secretive state would not back down over the launch, despite the urging of countries across the globe and the threat of the withdrawal of much-needed food aid.

"They are confident that China will ultimately support them if they launch the satellite" despite international pressure, he told reporters ahead of the publication of the "East Asian Strategic Review" published by NIDS.

Washington has asked Beijing to rein in its errant ally, but the response so far has been muted.

China has expressed concern over the planned launch and urged "restraint" on all sides following pronouncements from some countries -- including Japan -- that any rocket could be shot down.

Pyongyang insists it has the right to launch what it maintains is a satellite for peaceful purposes, but the US and its allies say any launch would contravene UN sanctions aimed at curbing North Korea's missile programme.

The study, being published Friday, is an annual venture commissioned by Japan's Defence Ministry, which influences national defence policy.

It warns improvements in nuclear technology, coupled with the change in leadership that has seen the untested Kim Jong-Un take control in Pyongyang, have increased the danger for the region.

Experts have said the communist state may have succeeded in miniaturising its nuclear weapons, raising alarm bells among defense analysts.

"If this leads to North Korea gaining the ability to mount its nuclear warheads on ballistic missiles, it would pose a major threat to countries in the region, including Japan," the study said.

Pyongyang's insistence on pressing ahead with uranium enrichment, along with an increase in the target range of ballistic missiles, is also alarming, it added.

"As of January 2012, the new regime under party and military leader Kim Jong-Un... may resort to missile firings, nuclear tests and other provocations to boost its national prestige and strengthen the regime's foundations," the report said.

"The security situation in the Korean Peninsula therefore remains as unpredictable as ever," it said.

North Korea had begun fuelling its rocket for launch on April 12 or 13, a media report said Thursday, the day after the United States suspended planned food aid.

North Korea insisted this week it would go ahead with what it calls a peaceful satellite launch, snubbing a call from US President Barack Obama to drop the plan and accusing him of a "confrontational mindset".

The United States has suspended plans to send food aid to North Korea, saying it has broken a promise to halt missile launches and cannot be trusted to give the aid to those who need it, a Pentagon official said Wednesday.

Under a deal reached last month, North Korea had agreed to a partial nuclear freeze and a missile test moratorium in return for 240,000 tonnes of US food aid.

But, said Akutsu, there would be a silver lining if North Korea goes ahead with the launch.

"It will... provide an opportunity for us to observe progress in North Korea's missile technology," he said.