The move by Japan to send its Izumo helicopter carrier through the waters where Beijing has been expanding strategic control signals clear fears even among close US allies about Donald Trump's commitment to Asia, leading Australian experts said.

Ms Bishop, when asked for her views on the reports of Japan's planned naval transit, said: "The Australian government supports the right of all nations and their vessels to traverse international waters according to international law."

Overnight, Chinese Foreign Ministry spokeswoman Hua Chunying said China had seen the media reports about Japan's actions, but hadn't heard Japan's official explanation.

China wasn't worried if the Japanese ship was going on a regular visit to several countries and passing "normally" through the South China Sea, she said.

However, she noted that if Japan had other intentions "it should be regarded as a different matter".

Japan had recently sown discord and "fanned the flames" in the South China Sea issue, she said, but China hoped Japan was capable of contributing to the peace and stability of the area.

Strategic scholars meanwhile said the Trump administration needed to do more than make vague, reassuring statements if it is to calm nervous Asian nations - including Australia - who worry the US might withdraw from the region.

US acting Assistant Secretary of State Susan Thornton said on Monday night that the Trump administration would ditch the Obama-era US language of a "pivot" or "rebalance", which described a long-term plan to focus more military, diplomatic and economic attention on the Asia-Pacific region.

Ben Schreer, the head of Macquarie University's Department of Security Studies and Criminology, said Japan's decision to send the 248-metre long Izumo through the South China Sea reflected Tokyo's wish to signal to Washington that it would do more militarily in Asia.

This in turn was aimed at encouraging the US to stay involved, underscoring the nervousness among Washington's allies in Asia, including Australia, that the superpower would pull back.

"[The Izumo] is their most powerful warship so it sends a message and it sits within [Japanese Prime Minister Shinzo] Abe's policy of signalling to the Americans that Japan is within limits willing to do more ??? and getting American reassurance in return," Professor Schreer said.

Ms Bishop in a speech in Singapore on Monday said that many countries in Asia were in a "strategic holding pattern" as they waited to see whether the US would remain committed to the region. She called on the Trump administration to "play an even greater role as the indispensable strategic power in the Indo-Pacific".

Euan Graham of the Lowy Institute said the Japanese move was "a bold move" but how bold would depend on whether it sailed with US naval ships nearby or within disputed waters.

That would be "a significant up-tempo shift - one that would inevitably raise expectations of Australia", he said.

Andrew Shearer, a former adviser on national security to Tony Abbott and John Howard, now with the US Centre for Strategic and International Studies, said Japan was taking another step in a stronger security posture under Mr Abe, which came on top of a decision to conduct exercises with the US in the South China Sea.

"To me, this underlines Japan's commitment to upholding freedom of navigation, its growing concern about tensions in the South China Sea - an international waterway that is vital to its economy as well as Australia's - and the extent to which other countries in the region are anxious about China's growing assertiveness.

Dr Graham and Professor Schreer said it was striking that Ms Bishop had so pointedly highlighted the wait-and-see attitude in Asia about Mr Trump's commitment to Asia.

Ms Thornton's remarks about the pivot or rebalance being "a bumper sticker" that was used to describe "the Asia policy in the last administration" might be damaging because there was nothing so far to replace it, they said.

"Until the US does more to fill the policy void ??? scepticism is inevitable about how far the inner core around President Trump are willing to buy into those as US interests," Dr Graham said.

Professor Schreer said: "If it's not the rebalance en or the pivot, what is it? What remains of the engagement?"

Ms Bishop said of Ms Thornton's remarks that she was "encouraged during my recent meetings with the United States Administration, including with Vice-President Pence, Secretary of State Tillerson and National Security Adviser General McMaster that the US intends to remain engaged in the Asia Pacific region".

- with Kirsty Needham

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