A Chinese military frigate locked its weapon-targeting radar on a Japanese navy vessel on at least one occasion, Japan’s defence minister said Tuesday.

“On January 30, something like fire-control radar was directed at a Japan Self-Defense Maritime escort ship in the East China Sea. The defence ministry today confirmed radar for targeting was used,” Itsunori Onodera told reporters in Tokyo.

Onodera said a Japanese military helicopter was also locked with a similar radar a few days earlier.

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

The move is an apparent ramping 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.

On Tuesday Tokyo summoned China’s envoy in protest at the presence a day earlier of Chinese government ships in the waters around the islands.

No mention was made in the earlier 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.

“The foreign ministry summoned the Chinese ambassador over ships entering the waters near Senkaku islands,” said an official, referring to a chain claimed as the Diaoyus by Beijing.