The Japanese Coast Guard said eight surveillance vessels entered Japanese territorial waters around the disputed Senkaku Islands in the East China Sea, the Kyodo news agency reported.
CNN said the Chinese vessels entered the disputed waters after a group of Japanese activists chartered fishing boats and traveled to the disputed area. Xinhua, the official state news outlet in China, said the Japanese activists were "making trouble."
Japan said it warned the Chinese ships to leave its territorial waters immediately.
The uninhabited islands are under Japanese control but they are also claimed by China, which calls them the Diaoyu Islands.
"It is extremely regrettable and unacceptable that Chinese state ships continue to engage in intrusion," Chief Cabinet Secretary Yoshihide Suga said.
Relations between Japan and China have been frayed over conflicting claims to the islands. They were further roiled last week when 170 Japanese lawmakers and several cabinet ministers visited a controversial World War II Japanese memorial on a remote island. China considers the memorial a symbol of Japanese aggression during the war.
NASA celebrates Earth Day with #GlobalSelfie
Turkey considering to use pistachios to heat country’s first eco-city