Japanese foreign minister Yoshimasa Hayashi says the US has promised an "unwavering commitment" to defending his country, including in the case of southern islets disputed between Tokyo and Beijing.
Speaking at a news conference at the ministry in the capital, Hayashi said US Secretary of State Antony Blinken had assured of the support in a phone call on Saturday.
"Secretary Blinken stated that US commitment to defending Japan, including the application of Article 5 of the Japan-US Security Treaty for the Senkaku islands, was unwavering," he told reporters.
The uninhabited East China Sea islands of Senkaku lie about 220 km (135 miles) west of Chinese Taipei and are controlled by Tokyo but claimed by Beijing which calls them Diaoyu.
Tension between China and Japan escalated in 2012 when the Japanese government nationalized control over three of the Senkaku Islands in the East China Sea.
The uninhabited yet strategically-important island group has been under the Japanese administrative control since the reversion of Okinawa to Japan from US administrative rule in 1972.
China maintains that the islands are an inherent part of its territory and that it has indisputable sovereignty over them. The Japanese government, instead, regards them as a part of its Ishigaki Island.
China warns US not to support Taipei’s independence
China's foreign ministry said in a statement on Saturday that senior Chinese diplomat Wang Yi had told Blinken not to "send wrong signals" to pro-independence forces in Taipei.
Both officials were also said to have spoken about the virtual meeting that Chinese President Xi Jinping and US President Joe Biden would have on Tuesday.
The US secretary of state threatened on Wednesday that the United States and its allies would take unspecified "action" if China were to use force to alter the status quo over Taipei.
China has sovereignty over Chinese Taipei, and under the "One China" policy, almost all world countries recognize that sovereignty. The US, too, recognizes Chinese sovereignty over the island but has long courted Taipei in an attempt to unnerve Beijing.
The United States, which backs Taipei's secessionist president, also continues to sell weapons to the island in defiance of Beijing and in violation of its own stated policy.
'Inconceivable' for Australia not to join US to defend Taipei: DM
Meanwhile, Australian Defense Minister Peter Dutton said on Saturday that it would be "inconceivable" for Australia not to join the United States should Washington take action to defend Chinese Taipei.
"It would be inconceivable that we wouldn't support the US in an action if the US chose to take that action," Dutton told The Australian newspaper in an interview.
"And, again, I think we should be very frank and honest about that, look at all of the facts and circumstances without pre-committing, and maybe there are circumstances where we wouldn't take up that option, (but) I can't conceive of those circumstances."
China's military announced earlier in the week that it conducted a combat readiness patrol in the direction of the Taiwan Strait after a visit by a US congressional delegation to Chinese Taipei.
"China's been very clear about their intent to go into Taiwan and we need to make sure that there is a high level of preparedness, a greater sense of deterrence by our capability, and that is how I think we put our country in a position of strength," Dutton told the newspaper.
Tensions between Chinese Taipei, China and the US have been at their highest in decades.
China has been flying fighter jets close to Chinese Taipei while the US has reportedly had troops deployed in the territory for the past year training their people.
Last month, Biden said the United States would come to Taipei's aid if it were to come under attack from China, claiming it had a commitment to defend the self-ruled island.