Four Chinese surveillance ships entered Japanese territorial waters near the disputed islands in the East China Sea early Thursday morning, evoking protest from Japan’s Foreign Ministry, news agencies reported.

A spokesman for Japanese Coast Guard headquarters in Okinawa, Atsushi Takahashi said Japanese coast guard ships maneuvered close to the Chinese vessels and asked them to leave. But the patrol ships refused, saying the area was a Chinese territory, the Associated Press reported.

According to the Japanese Coast Guard, three of the vessels entered the waters about 22 kilometers south-southeast of Minami-Kojima, a disputed island, around 6:30 a.m. Thursday. A fourth vessel followed the route about an hour later.

Japan’s Foreign Ministry was quick to lodge its protest over the latest incursion with China’s ambassador in Tokyo Cheng Yonghua, saying Chinese Marine Surveillance ships were spotted within an area of 12 nautical miles.

Cheng responded saying the islands were part of China.

This is the sixth time Chinese patrol vessels have entered the disputed waters since the political standoff that began last month over the five main islands called Senkaku in Japanese and Diaoyu in Chinese, according to Japan’s Asahi Shimbun newspaper. The last occurrence was Oct. 3.

The decades-long sovereignty dispute between the two nations escalated Sept. 11 when Japan announced the signing of a contract worth 2.05 billion yen ($26 million) to buy three of the five main Senkaku islands from its private owner.

Chinese vessels reportedly maintain a close vigil over the Islands since Japan’s purchase of the islands. The four vessels involved in the latest incident are known to have been in the area since Oct. 20, according to Asahi Shimbun.