Austin will meet Ishiba and then Japanese counterpart Gen Nakatani on what is likely his last official trip to the region, as the key allies prepare for a new era shaped by US president-elect Donald Trump's "America First" policies.

The Pentagon chief on Monday described the threat posed by China in a speech aboard the aircraft carrier USS George Washington, which is stationed off Yokosuka south of Tokyo.

"The PRC is the only country in the world that has the intent and, increasingly, the capability to change the rules-based international order," he said, using the initials of its full name, the People's Republic of China.

"And so, you know, we want to see this region, this area remain open to freedom of navigation and the ability to fly the skies and international airways whenever we want to," Austin said.

"We're going to continue to work with our allies and partners to ensure that we can do just that."

Around 54,000 US military personnel are stationed in Japan, mostly in Okinawa, east of Taiwan. Increased military activity around the self-ruled island since Monday has fuelled speculation about drills by China, which claims Taiwan as its territory.

Austin also spoke about the situation in Syria, where Islamist rebels have unseated president Bashar al-Assad after decades of brutal rule.

"As this unfolds, there's a potential that elements in the area, such as ISIS, could try to take advantage of this opportunity and regain capability," Austin said, in comments echoed by US Secretary of State Antony Blinken.

Austin's trip to Japan is his 13th visit to the Asia-Pacific region as defense secretary, according to the Pentagon. Trump, who will be sworn in next month, hopes to replace him with Pete Hegseth, a former military officer and Fox News presenter.

Analysts predict that Trump's protectionism will mean less cash from Washington for security in the region, with investors betting on Japan upgrading its own military capacities.

The country is already in the process of doubling its military spending to the NATO standard of two percent of GDP.

South Korean President Yoon Suk Yeol's brief declaration of martial law last week, and the ensuing political turmoil, reportedly led to a planned Seoul leg being cut from Austin's itinerary.

This "significantly undercut" the value of his Asia trip, Daisuke Kawai, deputy director of the University of Tokyo's economic security research programme, told AFP, citing a lost last-minute chance for Washington to solidify its ties with the pair.

But the outgoing defence chief's trip remains a chance for Washington to assure its closest ally that it "won't abandon Japan even after Trump swings back to power", Kawai said.

Tokyo was also keen to strengthen ties to "leave Trump less room for a policy change", he added.

Austin likely sees the talks as a chance to press Japan to keep providing support for Ukraine, even after Trump has said he will "probably" reduce aid to the war-torn country.

Last month, Ishiba said he would have "frank discussions" with Trump, saying cooperation was vital to ensuring a "free and open Pacific".