US President Barack Obama vowed Thursday to defend Japan if China attacks over a tense territorial dispute, but also urged Beijing to step in to thwart North Korea's "dangerous" nuclear march.

In Tokyo on the first leg of his Asian tour, Obama stopped short however of fully endorsing Japanese sovereignty over disputed East China Sea islands, keeping one eye on Beijing where his trip is being weighed for any hint of US hostility.

Hopes that Obama's trip would revive the stalled Trans-Pacific Partnership (TPP) meanwhile looked doomed when there was little progress on one of the key sticking points -- US access to the Japanese auto market -- after his talks with Japan's Prime Minister Shinzo Abe.

After flying into a region boiling with tension, Obama carefully underlined US support for Japan's security, saying that islands at the centre of its feud 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 archipelago which Beijing calls the Diaoyus.

"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."

Obama added that "this is not a new position" and "there's no red line that's been drawn".

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

- Strategic ambiguity -

Along with Manila -- the fourth stop on Obama's tour -- Tokyo craved reassurance that Washington was prepared to support it if push comes to shove with Beijing over their separate sovereignty rows.

Obama did not spell out in detail what Washington would do if Beijing stormed the islands -- preserving some strategic ambiguity.

And though he stressed that the islands had been administered by Japan for years he added: "We do not take a position on the particular sovereignty of this piece of land or this rock."

Relations between Tokyo and Beijing are at their lowest point for years. Some observers warn they might come to blows over the islands, where ships from both sides lurk to press claims for ownership.

Abe's position on historical issues also annoys the Chinese, who accuse him of playing down Japanese atrocities. They are particularly upset by visits he and his cabinet ministers have paid to the Yasukuni Shrine, which honours war criminals among other fallen warriors.

Obama did not mention the issue publicly, but US officials say it is often raised in private in talks with the Japanese.

Obama also made a point of addressing North Korea, as fears mount that Pyongyang could stage another nuclear test in a grab for attention during Obama's tour of the region, which takes him to Seoul on Friday.

He said China's role in keeping its wayward ally in check was "critically important" after South Korea said heightened activity at the North's main nuclear test site could point to an imminent test -- its fourth.

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

In response to Obama's comments, China said it was working to avoid a crisis on the tinderbox Korean peninsula.

"We will by no means allow war or chaos to occur at the doorstep of China," foreign ministry spokesman Qin Gang said.

Qin dismissed Obama's stance on the Japan island dispute though.

"No matter what others say or do it cannot change in the slightest the basic fact that the Diaoyu Islands are China's inherent territory".

- 'Time for bold steps' -

Negotiators from Japan and the United States extended efforts to try to reach a deal over disputed access to Japan's auto and agricultural sectors under the proposed TPP deal.

The vast 12-nation pact would be a highlight of Obama's Asian legacy, but the wrangle between Tokyo and Washington threatens to derail wider TPP talks entirely.

"Now is the time for bold steps," Obama told Abe -- though admitted his host faced tough political choices over the deal.

Obama's best laid plans to divert US military and diplomatic resources to Asia are constantly being derailed by crises elsewhere -- and this trip is no exception.

He was forced to address the deepening crisis in Ukraine, accusing Russia of reneging on a deal reached in Geneva last week to de-escalate tensions.

Aside from the trouble boiling around him, Obama appeared to revel in a return to a city he first saw as a six-year-old boy.

He made time for some quiet reflection at the gracious Meiji Jingu, a huge shrine in central Tokyo dedicated to the Emperor Meiji, whose return to prominence in 1868 marked the birth of modern Japan.

He also played football with an advanced robot pioneered by Honda, to highlight US-Japanese technological ties.

Later Thursday Obama lauded more than half a century of peaceful ties with Tokyo amid the pomp of a state dinner hosted by Emperor Akihito.

"Though separated by a vast ocean, our peoples come together every day and in every realm," Obama said.

After South Korea, Obama's trip takes him to Malaysia and the Philippines.