US president Barack Obama has vowed to defend Japan in its dispute with China over islands in the East China Sea.

On the second day of his Asian tour, Mr Obama underlined US support for Japan, saying that islands at the centre of its bitter territorial dispute with Beijing are covered by a defence treaty that would oblige Washington to act if they were attacked.

"Article five (of the US-Japan security treaty) covers all territories under Japan's administration including (the) Senkaku islands," he said, referring to the East China Sea archipelago, which Beijing calls the Diaoyu islands.

"We don't take a position on final sovereignty determinations with respect to Senkaku, but historically they have been administered by Japan.

"We do not believe that they should be subject to change unilaterally, and what is a consistent part of the alliance is that the treaty covers all territories administered by Japan."

Mr Obama said "there's no red line that's been drawn" at a joint news conference after his summit with Japanese prime minister Shinzo Abe.

"This is not a new position, this is a consistent one," he said.

"We stand together in calling for disputes in the region, including maritime issues, to be resolved peacefully through dialogue."

Free trade bloc progress stalled

However, progress on the Trans-Pacific Partnership (TPP) - a proposed 12-nation free trade bloc - has stalled.

Mr Obama said on Thursday the two sides had still yet to find common ground, with high tariffs on Japanese agricultural products among the key sticking points.

"Now is the time for bold steps," he said.

The two leaders had hoped to be able to announce a breakthrough after lengthy negotiations aimed at shifting a log-jam over automotive and agricultural products, with powerful vested interests on both sides.

American and Japanese trade negotiators failed to resolve differences in time for Mr Obama and Mr Abe to agree to a two-way pact at the summit, but the two leaders said there has been progress and ordered their teams to keep working to narrow gaps in their trade talks.

Mr Abe has touted the TPP as key to "Third Arrow" reforms needed to generate growth in the world's third-biggest economy, along with eased monetary policy and fiscal spending.

Both sides have also stressed that the TPP would have strategic implications by creating a framework for business that could attract China to adhere to international rules.

But the talks have been stymied by Japan's efforts to protect politically powerful agriculture sectors such as beef, and disputes over both countries' automotive markets.

"Those are all issues that people are all familiar with and at some point have to be resolved. I believe that point is now," Mr Obama said, referring to restrictions on access to Japan's farm and auto sectors.

Call for China's help

Meanwhile, Mr Obama has urged Beijing to help stop North Korea from forging ahead with its nuclear program.

"China's participation in pushing the DPRK (North Korea) in a different direction is critically important," Mr Obama also told the press briefing.

"It is the most destabilising, dangerous situation in all of the Asia-Pacific region."

Mr Obama's comments come after South Korea said heightened activity at North Korea's main nuclear test site could point to an imminent test.

The US president says there are also opportunities to work with China, but has called on the Asian power to stick to international rules.

"What we've also emphasized, and I will continue to emphasize throughout this trip, is that all of us have responsibilities to help maintain basic rules of the world and international order, so that large countries, small countries, all have to abide by what is considered just and fair," he said.

Calls to raise human rights, corruption issues in Malaysia

After Japan, Mr Obama's trip to Asia will take him to South Korea, Malaysia and the Philippines.

The Mr Obama is due to arrive in Malaysia at the weekend.

Malaysia's opposition Democratic Action Party (DAP) spokesman, Tony Pua, has told Radio Australia's Asia Pacific program that while trade talks are likely to dominate Mr Obama's visit, the US leader should also raise issues of human rights, government corruption and poverty.

"I think it is important for Malaysians to hear from the president of the United States that he is concerned with human rights, he's concerned with corruption and he's concerned with abuse of power in Malaysia," Mr Pua said.

"Everything cannot be about commerce. If the United States believes in the founding fathers of the United States, that means that preservation of liberty, as well as human rights and all the past leaders whom the world looks up to coming from the United States, [then] people like Obama should put an effort to make a difference in this point."

Mr Pua says Mr Obama's snub of Malaysian opposition leader Anwar Ibrahim, who faces jail on a sodomy charge that is widely seen as politically-motivated, might diminish US credibility in the eyes of Malaysians.

"Malaysians expect more from the president of the United States, and not just the president of a country seeking commercial gains from a developing country," he said.

ABC/wires