A Chinese naval ship sailed in Japan's waters earlier this week off its southwestern prefecture of Kagoshima, the Defense Ministry said Friday, as security tensions between the two countries have shown no signs of easing.

It is the first time since July 2017 that the ministry has confirmed and disclosed a Chinese naval vessel's intrusion into Japanese waters. It is the fourth time in total for such an incursion.

The Japanese government conveyed concern about the vessel's action to China through diplomatic channels, said officials familiar with the matter.

But no order based on the Self-Defense Forces Law allowing the defensive use of weapons on the seas was issued, according to the officials.

The Chinese survey ship was spotted sailing off the southwestern prefecture on Wednesday and Thursday in the so-called contiguous zone outside Japan's territorial waters, the ministry said.

The ship heading toward Japanese waters from the contiguous zone south of Yakushima Island was spotted by a Maritime Self-Defense Force patrol plane around 8:40 p.m. on Wednesday, the ministry said.

The ship was then seen sailing west of Kuchinoerabu Island in the zone westward around 1:20 a.m. on Thursday.

Based on the pattern of sightings, the ministry determined that the vessel entered Japanese waters sometime between Wednesday night and early Thursday, though it did not specify when the intrusion happened.

China's increased maritime activities have become a major security concern for Japan and other countries in the region.

China frequently sends coast guard ships near the Senkakus, a group of Japan-controlled islets in the East China Sea. The uninhabited islets are claimed by China, which calls them Diaoyu.

On Friday, four Chinese coast guard ships also entered Japanese waters around the Senkakus, marking the first intrusion in the area since Oct. 20.

Japan's top government spokesman Hirokazu Matsuno said in a press conference it is "a violation of international law. It is truly regretful, and we cannot accept it."

Matsuno said Japan has lodged a protest with China because the islets are "no doubt an inherent territory of our country historically and based on international law."

He added Japan will enhance its surveillance of nearby waters.

On Friday, Japanese fighter jets were scrambled in response to two Chinese and two Russian bombers flying over the Sea of Japan and the East China Sea, the ministry said.

It has also recently confirmed that a total of three Chinese and Russian naval ships had entered the East China Sea through the Tsushima Strait in Japan's southwest.