The US and Japan have been conducting war games and joint military exercises in the event of a conflict with China over Taiwan, amid escalating concerns over the Chinese military’s assertive activity.

US and Japanese military officials began serious planning for a possible conflict in the final year of the Trump administration, according to six people who requested anonymity. The activity includes top-secret tabletop war games and joint exercises in the South China and East China seas.

Shinzo Abe, then Japanese prime minister, in 2019 decided to significantly expand military planning because of the Chinese threat to Taiwan and the Senkaku Islands in the East China Sea. This work has continued under the administrations of Joe Biden and Japanese prime minister Yoshihide Suga, according to three of the people with knowledge of the matter.

The US and Japan have become alarmed as China has flown more fighter jets and bombers into Taiwan’s air defence identification zone, including a record 28 fighters on June 15. The Chinese navy, air force and coast guard have also become increasingly active around the Senkaku, which are administered by Japan but claimed by China and Taiwan.

China insists that it wants to unify Taiwan with the mainland. While it says it wants peaceful unification, it has not ruled out the use of force to seize control of Taiwan.

“In many ways, the People’s Liberation Army drove the US and Japan together and toward new thinking on Taiwan,” said Randy Schriver, who served as the top Pentagon official for Asia until the end of 2019. “Assertiveness around the Senkaku and Taiwan at the same time drives home the issue of proximity.”

The US has long wanted Japan, a mutual defence treaty ally, to conduct more joint military planning, but Japan was constrained by its postwar pacifist constitution. That obstacle was eased, but not eliminated, when the Abe government in 2015 reinterpreted the constitution to allow Japan to defend allies that came under attack.

As the two allies started to bolster their joint planning, Japan asked the US to share its Taiwan war plan, but the Pentagon demurred because it wanted to focus on boosting planning between the two countries in phases. One former US official said the eventual goal was for the two allies to create an integrated war plan for Taiwan.

Two of the six people said the US military and Japanese self-defence forces had conducted joint exercises in the South China Sea that had been couched as disaster relief training. They have also held more military exercises around the Senkaku, which also helps prepare for any conflict with China over Taiwan, which is just 350km west of the islands.

“Some of the activities we’re training on are highly fungible,” said Schriver, adding that exercises such as an amphibious landing in a “disaster relief scenario” would be “directly applicable” to any conflict around the Senkaku or the Taiwan Strait.

Mark Montgomery, a retired admiral who commanded the USS George Washington aircraft carrier strike group and was director of operations at Indo-Pacific command between 2014 and 2017, said the Pentagon needed a “comprehensive understanding” of the support Japan could provide in the case of a conflict.

“As a crisis grows and Japan is potentially drawn in as a participant, the US will need to understand how Japan could support or enable US operations,” he added.

US and Japanese diplomats are examining the legal issues related to any joint military action, including access to bases and the kind of logistical support Japan could provide US forces engaged in a conflict with China.

In the event of a war over Taiwan, the US would rely on air bases in Japan. But that raises the odds that Tokyo would be dragged into the conflict, particularly if China tried to destroy the bases in an effort to hobble the US.

One official said the US and Japan needed to urgently create a trilateral sharing mechanism with Taiwan for information about Chinese naval and air force movements, especially around the Miyako Strait to the east of Taiwan which is covered by Japanese sensors from the north-east and Taiwanese sensors from the south-west.

“Some of that kind of data is shared between Taiwan and the US, and between Japan and the US. But we have no direct sharing trilaterally,” the official said. “You cannot start setting that up in the middle of a contingency. You have to do it now.”

Another official said the three nations had taken a small but important step in 2017 by agreeing to share military aircraft codes to help identify friendly aircraft.

Taiwanese officials and US and Japanese sources said co-operation had since risen significantly, driven by the growing awareness in Japan about the importance of Taiwan — which is 110km from Yonaguni, the westernmost island in the Japanese archipelago — for its own security.

“The Japanese government has increasingly recognised, and even acknowledges publicly, that the defence of Taiwan equates to the defence of Japan,” said Heino Klinck, a former top Pentagon official who oversaw military relations with Japan and Taiwan from late 2019 until the end of the Trump administration.

The Japanese defence ministry said Tokyo and Washington continued to update their joint planning following the 2015 revision of guidelines that underpin the military alliance, but declined to provide any detail. The Pentagon did not comment.

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Additional reporting by Robin Harding in Tokyo