The Wall Street Journal quoted officials as saying that the US and Japanese militaries have been working for more than a year on a plan to defend Taiwan in the event of a Chinese attack, as Russia and China prepare for joint exercises.
But officials said Japan's military would not take part in any military action against China.
Washington has urged Tokyo to consider a role for its military, such as tracking Chinese submarines around Taiwan, and talks also include supply routes, missile launch sites and evacuation plans.
Japan is ready to support the U.S. military by providing fuel and other supplies, but refuses to engage its military militarily with Washington if a war breaks out with Beijing over Taiwan.
Japanese leaders refuse to discuss any role in the war on Taiwan because of public opinion opposed to involvement in the conflict, and Tokyo's biggest concern is the risk of escalation.
China and Russia
Meanwhile, China's Defense Ministry announced on Saturday that the Russian armed forces will participate in joint military exercises organized by China in the Sea of Japan soon, following the conclusion of an expanded ASEAN conference in Jakarta on Friday that witnessed a frantic US rivalry.
According to the annual cooperation plan between the Russian and Chinese armed forces, the Russian military will soon send naval and air forces to participate in the "North-Interaction 2023" exercise organized by the Chinese military in the central part of the Sea of Japan, the Chinese Defense Ministry said.
The ministry said the drills will focus on maintaining the security of strategic sea lanes, improving strategic interaction between Chinese and Russian forces, enhancing the ability to jointly maintain regional peace and stability, and responding to various security challenges.