US Defense Secretary Lloyd Austin arrived in Japan on Sunday in his last trip to the Indo-Pacific as the Pentagon chief, as he seeks to cement the Biden administration's policy legacy of deepening regional alliances and partnerships.

Austin arrived at Yokota Air Base near Tokyo, kicking off the Asia swing that has excluded an anticipated stop in South Korea, another key US ally, in the aftermath of President Yoon Suk Yeol's botched martial law attempt last week.

During the trip through Wednesday, the secretary is scheduled to meet with Japanese Prime Minister Shigeru Ishiba and Defense Minister Gen Nakatani, visit a US aircraft carrier and meet with American troops, and observe a military exercise involving Australian forces, according to a senior US defense official.

"The secretary's trip is going to be highlighting how much the Department (of Defense) has achieved with Japan, a key treaty ally, over the last four years to advance peace, stability and prosperity in the Indo-Pacific," the official told reporters. "We are continuing to build on these accomplishments, and the next three days will give us a chance to see some of them in person and in implementation."

Austin plans to meet Ishiba and Nakatani separately on Tuesday to take stock of progress that the two countries have made in recent years and underscore the importance of the alliance, the official said.

Other events include his visit to the USS George Washington nuclear-powered aircraft carrier at the Yokosuka Naval Base and his observation of a US-Japan military exercise where Australian forces are participating for the first time.

The secretary's trip to Japan comes as Washington sees Tokyo as vital to its "new convergence" efforts to bring together US allies and partners in fit-for-purpose groupings to promote a shared vision for a "free and open" Indo-Pacific.

"New convergence of allies and partners, who share a vision for a free and open Indo-Pacific, and (of) the stronger and more resilient and more capable, networked partnerships is strengthening deterrence in the region, and Japan is emerging as a common denominator of that network," the official said.

The cancellation of Austin's visit to Seoul has added to concerns that the aftermath of the short-lived martial law imposition could affect security coordination between Seoul and Washington on North Korea's evolving nuclear and missile threats.

After the martial law episode, Seoul and Washington also postponed a session of the Nuclear Consultative Group, the allies' key nuclear deterrence platform, and a related exercise originally slated to take place in Washington last week.

US officials have expressed "grave concern" over the martial law declaration though they reiterated that America's commitment to the alliance with South Korea remains "ironclad." (Yonhap)