Japan said Monday it had protested to Paris over a French firm's sale of helicopter landing equipment to China, at a time of heightened territorial tensions between Tokyo and Beijing.

France-based naval defence firm DCNS has sold at least 11 sets of the equipment, which help helicopters land on ships in bad weather, officials said, sparking complaints in February.

"We have expressed our concerns," Chief Cabinet Secretary Yoshihide Suga told a news conference on Monday, while a foreign ministry official added the protest was made through the embassy in Paris after DCNS announced the sales.

An earlier report by the Asahi Shimbun said the protest came after Japan spotted a sales contract between DCNS and China for two of the marine patrol vessels Beijing frequently sends into waters around the contested islands.

The equipment is a large, perforated steel plate that gives purchase to a grappling hook sent out by the helicopter, which can then use the guideline to draw itself down to the deck.

Japan fears the equipment could boost China's underdeveloped helicopter landing technology and may pose a threat to Japan's control of the Senkaku islands, which China claims under the name Diaoyu, the Asahi report said.

In response, Paris told Tokyo that the transactions were outside the European Union's ban on arms exports to China, the report said.

Without commenting on the report, a foreign ministry official told AFP: "We'll continue conveying Japanese concerns to France on future diplomatic opportunities."

The long-running row over the islands intensified in September when Tokyo nationalised part of the chain, triggering fury in Beijing and anti-Japan demonstrations across China.

Beijing has repeatedly sent ships and aircraft near the islands and both sides have scrambled fighter jets, though there have been no clashes.

China military officials admit ship radar lock: report
Tokyo (AFP) March 18, 2013 - Senior Chinese military officials have admitted for the first time that a frigate locked its radar on a Japanese destroyer during the two nations' row over disputed islands, Kyodo News reported Monday.

In one of the more serious incidents in an escalating wrangle over ownership of the islands in the East China Sea, Tokyo said the Chinese vessel effectively had a Japanese ship in its sights earlier this year.

Meanwhile, three Chinese marine surveillance ships were seen entering the territorial waters extending 12 nautical miles off one of the islands at around 6:30 pm (0930 GMT), Japan's coastguard said.

Kyodo reported that the vessels left the area by around 9:30 pm.

State-owned Chinese vessels have intermittently cruised near the Tokyo-controlled Senkakus, which China claims as the Diaoyus, since Japan nationalised some of them last September -- at times inside territorial waters.

Beijing has consistently denied the allegation of the radar lock and has accused Tokyo of exaggerating the "China threat" in a bid to manipulate world public opinion against its giant neighbour.

But Kyodo News cited unidentified senior Chinese military officials as saying the weapons targeting had taken place.

The officials, including "flag officers" -- those at the rank of admiral -- told Kyodo it was an "emergency decision" and not a planned action and was taken by the commander of the frigate, the report said.

The Tokyo-datelined report said the comments were made recently but gave no specifics.

The radar incident marked the first time the two nations' navies have locked horns in the increasingly bitter island row.

The Chinese officials told Kyodo that on January 30 the frigate and the Japanese destroyer were three kilometres (two miles) apart in international waters 110 to 130 kilometres north of the outcrops, the report said.

The commander of the frigate directed his vessel's weapons-targeting radar, based on the Chinese military's rules of engagement, without seeking instructions from the fleet command or navy headquarters, Kyodo cited the Chinese officers as saying.

It was not known if the commander had been reprimanded, Kyodo said.

Tokyo has also charged that a Chinese frigate locked its radar on a Japanese helicopter in the middle of January.

China has denied the accusations and its defence ministry said on Monday that the truth was "very clear".

"The Japanese allegation of Chinese navy vessels targeting warships and airplanes of the Japanese Self-Defense Forces with fire-control radar does not fit the facts," it said in a statement faxed to AFP.

"The Japanese side speculates from time to time on this issue, discredits the Chinese military and misleads the international community with ulterior motives."

A later statement from the Chinese navy carried by China's official Xinhua news agency said the foreign media reports had been "maliciously concocted".

The statement said the reports were "fabricated out of thin air" and accused them of trying to damage the Chinese military's image, mislead international opinion and win sympathy.