Biden’s Secretary of Defense Lloyd Austin and his Japanese counterpart Nobuo Kishi spoke on a phone call early Sunday, according to Japan’s Defense Ministry.
The two diplomats agreed that Article 5 of the US-Japan security treaty, which obliges Washington to respond to an attack on Japanese-administered territory, applies to East China Sea islands as well.
Japan has long been locked in a dispute over a small group of uninhabited islands in the sea, known respectively as the Senkaku or Diaoyu islands.
China maintains that it has indisputable sovereignty over the islands, but the Japanese government regards them as a part of its territory.
Austin expressed opposition to China’s activities in the region, where both Chinese and Japanese ships conduct daily exercises regularly.
Former President Donald Trump had repeatedly criticized the decades-old treaty with Japan as “unfair.” He said that “if someone attacks Japan, we go after them and we are in a battle, full force, in effect. If somebody should attack the United States, they don't have to do that. That's unfair.”