Two Chinese Coast Guard ships pursued a Japanese fishing boat in its home waters near the disputed Senkaku Islands in the East China Sea, the Japanese coast guard said on Saturday.

Japan’s Nikkei Asian Review reported the events occurred on Friday and according to the Japanese account, it ordered the Chinese ships to leave Japan’s territorial waters. The guard deployed a patrol ship to defend the fishing boat, which had three crew members aboard at the time of the aggression; they were reportedly unharmed in the action.

On Friday afternoon, the Japanese coast guard documented the presence of four Chinese Coast Guard ships in Japanese waters surrounding the small, uninhabited islands. Approximately two hours after the vessels were spotted, two ships from the Chinese fleet reportedly chased the Japanese fishing boat, located roughly 7.5 miles southwest of one of the islets, named Uotsuri Island.

Japan’s coast guard deployed patrol ships to the area and issued a radio warning to the Chinese ships, upon which the Chinese ships initially dispersed, according to the South China Morning Post (SCMP), citing Japanese news agency Jiji Press.

Despite the warning, the next day, two of the four Chinese Coast Guard vessels entered Japanese territorial waters again on Saturday evening, the Nikkei reported, citing the Japan Coast Guard.

On Friday, Japanese diplomats protested the aggression in phone calls to Chinese embassies, a Japanese government source told the Nikkei.

Shigeki Takizaki, director-general of Japan’s Asian and Oceanian Affairs Bureau – part of the Ministry of Foreign Affairs – allegedly called China’s embassy in Tokyo on Friday to express disapproval of the antagonism. According to the source, Japan’s embassy in Beijing also called the Chinese Foreign Ministry to issue a protest of the encroachment.

On Friday, the Chinese Coast Guard issued a statement via Weibo, a Chinese social media platform, saying its ships had “patrolled the territorial waters around the Diaoyu [Senkaku] Islands,” the SCMP reports.

Beijing claims the islands, which it calls the “Diaoyu Islands,” and has been sending ships into the area in recent months in an attempt to exert control over the disputed territory, which is governed by Japan. The Senkaku islands are uninhabited and small, but nearby undersea oil reserves make them valuable territory. The islands are also strategically beneficial for Japan’s military defenses in the East China Sea. Nearby Taiwan also claims the islands.

Chinese ships last entered Japanese territorial waters surrounding the Senkaku Islands on April 17, according to the Nikkei’s report.

In late February, Japan’s defense ministry said it was considering upgrading its missile defense capabilities in the East China Sea in response to increasing aggression from China in the disputed zone.