Austin's visit will likely be his last official trip to the region, as the United States and Japan prepare for a new era shaped by US President-elect Donald Trump's protectionist "America First" policies.

"These are very dynamic times," Austin told Ishiba on Tuesday. "May our alliance remain the cornerstone of peace and stability in this region for the foreseeable future."

Ishiba thanked Austin for his "dedicated efforts... (in) strengthening our bilateral alliance over the years".

The Pentagon chief had 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 official 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."

Austin will also meet his Japanese counterpart Gen Nakatani on Tuesday.

Around 54,000 US military personnel are stationed in Japan, mostly in Okinawa, east of the self-ruled island of Taiwan.

On Tuesday, Taipei said China, which claims Taiwan as its territory, was deploying dozens of ships in its biggest maritime mobilisation around the island in years.

- Protectionism -

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," he said, using another name for the Islamic State group.

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.