A Chinese military frigate locked weapons-targeting radar on a Japanese navy vessel, Tokyo's defence minister said Tuesday, in an apparent upping of the stakes in a bitter territorial row.

The move, described by the minister and a Japanese analyst as "dangerous", marks the first time the two nation's navies have locked horns in a dispute that has some commentators warning about a possible armed conflict.

"On January 30, something like fire-control radar was directed at a Japan Self-Defense Maritime escort ship in the East China Sea," Itsunori Onodera told reporters in Tokyo.

"The defence ministry today confirmed radar for targeting was used."

Onodera said a Japanese military helicopter was also locked with a similar radar on January 19. He did not specify whether the helicopter was airborne or on the deck of a ship at the time.

Officials said on both occasions the targeting had lasted "minutes".

"Directing such radar is very abnormal," he said. "We recognise it could create a very dangerous situation if a single misstep occurred.

"We will seek the Chinese side's self-restraint from taking such dangerous action."

The move is a ratcheting-up of an already tense situation in the East China Sea, where Asia's two largest economies are at loggerheads over the sovereignty of an uninhabited island chain.

Hisao Iwashima, a Japanese defense analyst formerly of Japan's National Institute for Defense Studies, said Beijing needed to answer for what its navy had done.

"It could have been a test but it the Japanese side wouldn't have known if it was, or if it would lead to a launch," he told AFP.

"The Chinese side is responsible for explaining why it took such potentially dangerous action."

Also Tuesday Tokyo summoned China's envoy in protest at the presence a day earlier of Chinese government -- but not military -- ships in the waters around the Tokyo-controlled Senkakus, which Beijing claims as the Diaoyus.

No mention was made in the announcement of the actions of any Chinese military vessels. It was not believed that the military ships had been in what Japan considers its waters at the time of the episode.

"The foreign ministry summoned the Chinese ambassador over ships entering the waters near Senkaku islands," said an official.

Two maritime surveillance boats had spent around 14 hours in the area, one of the longest periods they have been present, the coastguard said.

Chief Cabinet Secretary Yoshihide Suga told a news conference the actions were "absolutely unacceptable", adding Ambassador Cheng Yonghua had been called to the foreign ministry to hear Japan's "strong protest".

Chinese ships have repeatedly ventured into the waters, in what observers say is Beijing's bid to create a "new normal" in which Tokyo does not have effective control over the archipelago.

A Chinese government plane entered the chain's airspace in December, setting off sorties by Japanese fighter jets.

In recent weeks both countries have dispatched military planes, which on at least one occasion have shadowed each other, although there have been no clashes.

The radar targeting announced Tuesday has not previously been reported and is believed to be the first time the two navies have crossed swords, albeit at a distance and without a shot being fired.

But analysts say the ramping up of rhetoric -- Japan's new nationalist premier has maintained his robust pre-election stance on China -- and the more frequent confrontations raise the risk of an accidental armed conflict.

On a Saturday visit to Okinawa, Prime Minister Shinzo Abe vowed to defend Japan against "provocations".

His government has approved a rise in defence spending for the first time in over a decade, with some of the cash earmarked for beefing up defence of the contested island chain.

Japan's already well-equipped coastguard said last week it would create a special unit over three years with 10 new large patrol boats, two helicopter carriers and a 600-strong force dedicated to overseeing Japanese territory in the East China Sea.

Abe's posturing, which plays well with his powerbase, has, however, been accompanied by apparent olive branches offered to Beijing. In a television interview he suggested China and Japan needed to hold a summit and has sent two emissaries to Beijing in recent weeks.