Japan scrambled eight fighter jets on Thursday after a Chinese state-owned plane breached its airspace for the first time, over islands at the centre of a dispute between Tokyo and Beijing.
It was the first incursion by a Chinese state aircraft into Japanese airspace anywhere since the country's military began monitoring in 1958, the defence ministry said.
The move marks a ramping-up of what observers suggest is a Chinese campaign to create a "new normal" -- where its forces come and go as they please around islands which Beijing calls the Diaoyus, but Tokyo controls as the Senkakus.
It also comes as ceremonies mark the 75th anniversary of the start of the Nanjing Massacre, when Japanese Imperial Army troops embarked on an orgy of violence and killing in the then-Chinese capital.
F-15 jets were mobilised after a Chinese Maritime Surveillance aircraft ventured over the islands just after 11 am (0200 GMT), chief cabinet secretary Osamu Fujimura told reporters.
"It was a fixed-wing Y-12 aeroplane belonging to the Chinese State Oceanic Administration. We confirmed that this aeroplane flew in our country's airspace," he said.
"It is extremely regrettable. We will continue to resolutely deal with any act violating our country's sovereignty, in accordance with domestic laws and regulations."
The Y-12 is a twin-turboprop.
Japan mobilised eight F-15 jets and an E2C early-warning aircraft, the Asahi Shimbun reported, citing a defence ministry source. But the incident appeared to have passed off without any direct confrontation.
In Beijing, China's foreign ministry termed the flight as normal.
"China's maritime surveillance plane flying over the Diaoyu islands is completely normal," said spokesman Hong Lei.
"China requires the Japanese side to stop illegal activities in the waters and airspace of the Diaoyu islands," Hong said, adding they were "China's inherent territory since ancient times".
The coastguard said its regular patrol had spotted the plane.
"At about 11:06 am today, a patrol boat from the Japan Coast Guard confirmed the flight of a fixed-wing aeroplane, which belongs to the Chinese Oceanic Administration, in our country's airspace around Uotsuri Island. It was confirmed at a point about 15 kilometres south of Uotsuri," said a statement.
"The patrol boat immediately informed the fixed-wing aircraft: 'Fly without intruding into our country's airspace'. It replied to the effect that 'this is China's airspace'."
The State Oceanic Administration is part of the Ministry of Land and Resources. Its roles include law enforcement in Chinese waters.
Chinese government ships have moved in and out of waters around the islands for more than two months, but there have been no reports of any airborne action.
Four maritime surveillance vessels were logged there earlier in the day, the coast guard said, adding it had ordered them to leave.
Such confrontations have become commonplace since Japan nationalised the East China Sea islands in September, a move it insisted amounted to nothing more than a change of ownership of what was already Japanese territory.
But Beijing reacted with fury, with observers saying the riots that erupted across China had at least tacit backing from the Communist Party government.
China says 300,000 civilians and soldiers died in a spree of killing, rape and destruction in the six weeks after the Japanese military entered Nanjing on December 13, 1937.
Some foreign academics put the number of deaths lower, including China historian Jonathan Spence who estimates that 42,000 soldiers and citizens were killed and 20,000 women raped, many of whom later died.