Tokyo, Japan - China's "coercive behavior" threatens regional stability, US Defense Secretary Lloyd Austin said Tuesday, after Taiwan said Beijing was carrying out its biggest maritime mobilisation around the self-ruled island in years.

The Pentagon chief is in Tokyo on what will likely be his last official Asia-Pacific trip, as the US and Japan prepare for the incoming administration of President-elect Donald Trump.

Austin's nominated replacement is Pete Hegseth, a former Fox News host with a deeply controversial record.

"Now we're clear-eyed about the challenges to peace and stability in this region and worldwide," Austin said as he met his Japanese counterpart Gen Nakatani on Tuesday evening.

China Taiwan goes on high alert after China's military restricts airspace

"That includes coercive behavior by the People's Republic of China in the East China Sea and the South China Sea and elsewhere in the region," Austin said.

"It includes Russia's reckless war of choice in Ukraine, and it includes the DPRK (North Korea) support for Moscow's war, as well as its other destabilizing and provocative activities," he added.

Austin, who is on his 13th trip to Asia as defense secretary, vowed that "America's extended deterrence commitment to Japan and to the Republic of Korea is ironclad," adding that the "US-Japan alliance has never been stronger".

In his remarks, he did not directly mention Taiwan, the self-ruled island that China claims.

Earlier in the day, a senior Taiwanese security official said nearly 90 Chinese naval and coast guard ships were in waters along the so-called first island chain, which links Japan's Okinawa, Taiwan, and the Philippines.

China regards Taiwan as its territory and has not ruled out using force to bring it under its control.

At the meeting on Tuesday, Japan's defense minister said the regional security situation was "growing ever more severe."

Nakatani also praised Austin for his initiative in "strengthening and cementing the deterrence of the Japan-US alliance."

Around 54,000 US military personnel are stationed in Japan, mostly in Okinawa, east of Taiwan.

China China's Xi Jinping extends olive branch to US in statements on looming trade war

"These are very dynamic times," Austin told Japanese Prime Minister Shigeru Ishiba at a separate meeting earlier on Tuesday. "May our alliance remain the cornerstone of peace and stability in this region for the foreseeable future."

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.