A Chinese navy frigate trained its missile radar on a Japanese Self-Defense Forces vessel in the East China Sea last week, Japan's defense minister said Tuesday.

Itsunori Onodera also said that it is suspected that on Jan. 19 another Chinese frigate also trained its missile radar on an SDF helicopter in the same area.

Although the minister refused to give the precise location of these incidents, it is suspected that they may well have taken place near the Senkaku/Diaoyu islands, sovereignty over which is disputed by Tokyo and Beijing.

The Japanese government has filed formal protests with Beijing over the incidents, Onodera said at a press conference.

This type of incident is "unusual," he said, adding that such situations could "entail serious danger."

Since September 2012, when Japan bought three of the five islands in the uninhabited archipelago in the East China Sea, relations between the countries have worsened.

On Tuesday morning, the Japanese Foreign Ministry summoned the Chinese ambassador in Tokyo to protest over what it considers a new "intrusion" by Chinese vessels into the waters around the archipelago.

Yesterday's incident was the 25th occasion on which Chinese vessels have entered the waters around the islands since the Japanese government bought them.

The area where the islands are located has a large number of commercially exploitable fish and could contain important hydrocarbon reserves. EFE