Washington and Beijing are at loggerheads over a raft of issues, from chips to tariffs, but both have been alarmed by the growing assertiveness of China's military in the Pacific.

China claims self-ruled Taiwan as its own and has vowed to seize the island, by force if necessary, ramping up pressure including with major military drills in the Taiwan Strait.

US Rear Admiral Carlos Sardiello called the joint exercises -- involving around a dozen US ships, including two aircraft carriers -- "a great rehearsal opportunity for us."

"Our operations here reflect an assertion of the maritime rights of all nations in accordance with international law to freedom of navigation and air operations, anywhere that is allowed," Sardiello told reporters on the bridge of the USS Carl Vinson.

"Our highly trained sailors can operate in these complex, contested domains and be lethal and survivable and execute the mission, regardless of what the threat is," he said.

Japanese authorities are reportedly stepping up contingency planning for evacuating tens of thousands of people from its islands near Taiwan in the event of a military emergency.

Japanese and Chinese vessels have regularly been involved in tense incidents in disputed areas, in particular the Senkaku islands in the East China Sea, known by Beijing as the Diaoyus.

North Korea is also jangling nerves in Japan with tests of cruise and ballistic missiles, including one in November that flew over Okinawa carrying a military satellite.

Japan, South Korea and the United States have deepened their trilateral ties, including with a three-way summit in August at Camp David hosted by US President Joe Biden.

Tokyo and Seoul have also sought to improve relations with Beijing, with the three countries' foreign ministers agreeing in November to accelerate efforts to organise a three-way summit soon.