Secretary of State John Kerry vowed that the United States would defend Japan against attack including over islands claimed by China as tensions boil between the Asian powers.
Kerry, who said he would visit China next week, met in Washington with Japanese Foreign Minister Fumio Kishida on Friday and reaffirmed the 1960 treaty that commits the United States to protect its ally against attacks.
"That includes with respect to the South China Sea," he said, before correcting himself to say the East China Sea, where China and Japan have conflicting claims.
Fears of conflict rose in November when China imposed an air defence identification zone (ADIZ) over much of the East China Sea. Beijing says it now requires notification from planes crossing the island group, which is known as the Senkakus in Japan and the Diaoyu Islands in China.
"The United States neither recognises nor accepts China's declared East China Sea ADIZ and the United States has no intention of changing how we conduct operations in the region," Kerry said.
The United States and its allies are increasingly concerned that China will take similar action in the South China Sea, where the Philippines in particular has voiced worries about Beijing's maritime claims.
Kishida, for his part, extended an invitation for US President Barack Obama to make a state visit to Japan.
Diplomats say Obama is likely to visit Japan on an April tour of Asia, although Kerry is not expected to stop in Tokyo on his forthcoming trip. Kishida was visiting Washington after a rare open disagreement between the two allies.
The United States voiced disappointment in December when Prime Minister Shinzo Abe paid a pilgrimage to the Yasukuni Shrine, which honours 2.5 million Japanese war dead including convicted war criminals.
Abe's visit outraged China and also fellow US ally South Korea.
Kishida told Kerry that Japan valued its relationship with South Korea despite their "difficult issues," saying the two democracies needed to work together in the face of nuclear-armed North Korea.
"Going forward, we will make tenacious efforts in order to build a co-operative relationship with the Republic of Korea from a broad perspective," Kishida said.
Kishida also highlighted progress under Abe in relocating the US air base at Futenma within the island of Okinawa.