Japan's coastguard has detained a Chinese fishing boat within Japanese waters, China's Xinhua news agency said, citing the Chinese consulate general in Fukuoka, a city in southwestern Japan.
The captain of the boat, registered in the southeastern Chinese province of Fujian, and two crew members have been brought to the southern Japanese city of Kagoshima for questioning, Xinhua said on Sunday.
It said the captain had admitted he was in Japanese waters.
A spokesperson for the Japanese embassy in Beijing could not be reached for comment late on Sunday evening.
The Chinese fishing fleet tends to range far into the waters east of China to offset depleted stocks closer to shore.
The incident came as China transferred two destroyers and nine other ex-navy vessels to its maritime surveillance fleet, reports said Monday, a move designed to beef up its position in bitter territorial rows with Japan and other neighbours.
Beijing renovated the ships and transferred them to surveillance operations to "alleviate the insufficiency of vessels used to protect maritime interests", said a report on Tencent, one of China's major news portals.
China is embroiled in a maritime dispute with Japan that has seen tensions between the two Asian giants, the world's second- and third-largest economies, at times reach fever pitch.
It is also engaged in a simmering row with its southern neighbours over its claim to vast swathes of the South China Sea.
Beijing has been sending maritime patrol vessels into waters around the East China Sea islands - which it claims as Diaoyu and which Japan controls and calls the Senkaku - since Tokyo nationalised the chain in September.
China is apparently seeking to prove it can come and go in the area at will and on Monday a pair of Beijing's ships were spotted in the waters, according to Japan's coastguard, in the latest perceived incursion.
Two of Beijing's newly-refurbished vessels are destroyers, with one each to operate in the East and South China Seas, with the others including tugs, icebreakers and survey ships, according to the Tencent report.
It was not clear whether it was the first time the maritime surveillance fleet has acquired destroyers, or when the transfers took place.
The report was first published in the International Herald Leader, a Chinese-language newspaper linked to Beijing's official news agency Xinhua, and the author said the operation had been significantly strengthened.
"The maritime surveillance team's power has been greatly strengthened and its capacity to execute missions sharply improved, providing a fundamental guarantee for completing the currently arduous task to protect maritime interests," wrote Yu Zhirong, of the government's Research Centre for Chinese Marine Development.
Since 2000 the maritime surveillance fleet, which is tasked with "protecting China's interests and executing law enforcement missions", has also received a total of 13 new vessels, the report said.
Daily patrols have been stepped up from six vessels before the disputes heated up to "more than 10" Yu said, adding authorities planned to build another 36 surveillance ships by 2015.
A Chinese plane overflew the islands in the East China Sea earlier this month, in what Japan said was the first time Beijing had breached its airspace since at least 1958. Tokyo scrambled fighter jets in response.
Yu added in the report: "I believe Chinese maritime surveillance authorities will build and buy many ships and planes in the future with strong capabilities and advanced equipment."