China has transferred two destroyers and nine other ex-navy vessels to its maritime surveillance fleet, reports said Monday, as it moves 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", Tencent, one of China's major news portals, reported.

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 the 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 three of Beijing's ships were spotted in the waters around the islands, according to Japan's coastguard, in the latest perceived incursion.

Late on Monday the Kyodo news agency reported that Tokyo is considering introducing US spy drones to boost surveillance of its territorial waters near the Senkaku islands.

The Japanese defence ministry hopes to introduce the unmanned Global Hawk aircraft by 2015 "in a bid to counter China's growing assertiveness at sea, especially when it comes to the Senkaku Islands", the news agency said, citing unnamed government officials.

Two of Beijing's newly refurbished vessels are destroyers, with one each to operate in the East and South China Seas, while the others include tugs, icebreakers and survey ships, according to the Tencent report.

The destroyers, the Nanjing and Nanning, each have a displacement of 3,250 tonnes and a top speed of 32 knots, according to sinodefence.com, an independent UK-based website.

It said that during their time in the navy they were equipped with 130mm guns with a range of 29 kilometres (18 miles), anti-ship missiles and other weapons.

The Nanjing went into service in 1977 and the Nanning in 1979. Both retired this year from the Chinese navy, previous domestic media reports said.

It was not clear whether it was the first time the maritime surveillance fleet has acquired destroyers, or when the transfers took place.

China's foreign ministry declined to comment when asked about the destroyers at its regular briefing on Monday.

Officials at the defence ministry and headquarters of the China Marine Surveillance were not immediately available to comment when contacted by AFP.

The transfer 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 given significantly more capacity.

"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 that 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."