Chief Cabinet Secretary Yoshihide Suga said Monday that a Chinese aircraft carrier's voyage in the Pacific shows Beijing's expanding military capability and that Japan is closely monitoring the vessel.
The Self-Defense Forces will take steps for warning and surveillance activity in the area, Suga told a news conference.
China said the Liaoning, its first aircraft carrier, is carrying out routine exercises in the Western Pacific.
The carrier was part of a task force spotted sailing in the East China Sea by a Maritime Self-Defense Force destroyer and airplane about 110 km northeast of Miyako Island at around 10 a.m. Sunday, Suga said.
It marks the first time the MSDF has confirmed a Chinese aircraft carrier advancing toward the Pacific Ocean, Suga said.
The mission appears aimed at sending a message to U.S. President-elect Donald Trump, who has questioned whether Washington should continue to adhere to its nearly four-decade policy of recognizing Taiwan as part of "one China."
Separately Monday, three Chinese Coast Guard ships briefly entered Japanese waters around the Senkaku Islands in the East China Sea, the Japan Coast Guard said.
The vessels were spotted entering territorial waters around the group of Chinese-claimed uninhabited islets at around 10:40 a.m. They left after about 1½ hours, the Coast Guard said.