Campbell made the remarks in a virtual speech, pointing out that such partnerships have never carried more significance than now as the world faces many opportunities and challenges at the same time, Yonhap news agency reported.

"Both alliances have transformed from primarily security-focused relationships to truly comprehensive global partnerships, with impacts reverberating far beyond the Indo-Pacific," Campbell said in a prerecorded keynote speech during a forum hosted by the Asan Institute for Policy Studies, a Seoul-based think tank.

Campbell cast the series of bilateral and trilateral summits the US has had with South Korea, Japan, and separately with Japan and the Philippines as a clear demonstration of the "not just historic but unrivalled" level of commitment from Washington and its allies in the Indo-Pacific.

"As President Biden has made clear, our partners and allies are critical if we are to be successful in seizing these key opportunities and maintaining our readiness to confront the most pressing challenges of the 21st century," Campbell said.

"Our bilateral and trilateral bonds with the Republic of Korea and Japan will define the future of Asia and grow our collective prosperity and security," he said.

On the trilateral cooperation with Seoul and Tokyo, Campbell noted that such commitment was made possible by the leaders of the two Asian allies, which led to a dramatic improvement in their bilateral relations.

"We wouldn't be here without the tremendous courage that President Yoon (Suk Yeol) and (Japanese) Prime Minister (Fumio) Kishida have demonstrated in bringing Seoul and Tokyo closer together," Campbell said.

The Biden administration's Indo-Pacific strategy focuses on creating a "lattice-fence" structure of alliances, Campbell said, referring to trilateral or wider groupings of a handful of like-minded countries.

"We understand the tremendous importance of working together trilaterally and multilaterally and networking our alliances," he said. "We're creating a lattice fence arrangement with intertwined, overlapping and interlocking engagements."

The "latticework" alliances are compared with the "hub and spokes" alliances that centre around each country's close security bond with the US but not with each other.

The US AUKUS security partnership with Australia and Britain and the Quad security dialogue with Australia, India and Japan are cited as latticework alliance partnerships in the Indo-Pacific region -- groupings largely seen as aimed at keeping an assertive China in check.

Campbell also pointed out that modernising the alliances is about sharing the resolve to address regional threats, including North Korea.

"The people of the Republic of Korea and Japan know too well the danger to peace and stability posed by the DPRK, and we are absolutely united in confronting that shared threat," he said, referring to the South by its official name.

DPRK stands for the Democratic People's Republic of Korea, the North's official name.