TOKYO (AP) — Japan protested to Beijing after spotting Chinese and Russian warships just outside its territorial waters around the disputed East China Sea islands on Monday.

Japan’s Defense Ministry said a Chinese frigate sailed inside the “contiguous zone,” just outside Japanese territorial waters around Senkaku Islands, which Beijing also claims and calls the Diaoyu, for several minutes Monday morning.

The Chinese warship’s presence was confirmed about 40 minutes after a Russian frigate had entered the waters for more than an hour, the ministry said.

It was not immediately clear what was behind the latest Chinese-Russian military activity in the area. Japanese defense officials mentioned a possibility that the ships may have been there to avoid a typhoon.

Deputy Chief Cabinet Secretary Seiji Kihara said Japan lodged a protest expressing “grave concern” to Beijing over the incident.

“The Senkaku islands are an inherent part of Japan’s territory historically and under international law. The government will deal with the matter calmly but firmly to protect the Japanese land, territorial waters and air space,” Kihara said.

There was no violation of the territorial waters, he said.

In Beijing, China justified the frigate’s entry and criticized Tokyo’s protest. Chinese Foreign Ministry spokesperson Zhao Lijian said the islands are Chinese territory.

“The activities of Chinese vessels in waters nearby are lawful and justified,” he said at a regular news conference. “Japan has no right to make such irresponsible remarks.”

Japan sees China’s increasingly assertive military activity in the East and South China seas as a threat to regional stability. Toyo is especially sensitive to Chinese activities near the disputed islands.

Kihara said that Monday’s incursion into the contiguous zone, which is between the territorial sea and the wider exclusive economic zone, by the Chinese warship was a fourth such event since June 2016.

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Associated Press writer Ken Moritsugu in Beijing contributed to this report.

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