A Chinese plane has been spotted above small islands controlled by Tokyo but claimed by Beijing, officials in Japan said.
It is the first time a Chinese aircraft has allegedly violated airspace over the islands amid a growing territorial row.
Japan issued a formal protest but China said it was merely carrying out a normal operation.
The chief government spokesman said the Chinese plane entered Japanese airspace in the morning.
The defence agency said four Japanese F-15 jets headed to the area but the Chinese plane, a non-military Y-12 aircraft, was nowhere to be seen by the time they got there.
The foreign ministry said a formal protest was sent to the Chinese government through the embassy in Japan.
The islands known as Senkaku in Japanese and Diaoyu in Chinese have been at the centre of a territorial dispute. The purchase of the islands by the Japanese government earlier this year from private Japanese owners set off massive anti-Japanese rallies in China.
“I want to stress that these activities are completely normal. The Diaoyu and its affiliated islands are China’s inherent territory since ancient times,” Chinese foreign ministry spokesman Hong Lei said of the plane.
“China requires the Japanese side stop illegal activities in the waters and airspace of the Diaoyu islands.”
It was the first time a Chinese plane had entered Japanese airspace over the disputed islands, as the two previous reported violations were by other nations, in 1979 by a Soviet plane and in 1994 by a plane from Taiwan, defence agency official Takashi Inoue said.
US assistant secretary of state Kurt Campbell, while noting he did not yet know the specifics of the latest incident, urged calm, while noting the US does not “take a position on the sovereignty of these islands”.
“The security treaty between the United States and Japan applies to any provocative set of circumstances,” he told reporters at the US embassy in Kuala Lumpur during an Asian tour. “We are encouraging all sides to take appropriate steps so that there will be no misunderstanding or miscalculation that could trigger an environment that would be antithetical to peace and stability.”
Japanese ships have been patrolling the area around the islands in the East China Sea, on the look-out for approaching Chinese ships. Chinese ships have darted in and out the waters in that area in recent months.
Some analysts say China-Japan tensions are at their highest in years. Japanese exports have tumbled since relations soured.