Chinese coast guard ships entered Japan’s waters near the Senkaku Islands, where the two have had a long-running territorial dispute, Japan's coastguard said on Wednesday.
Although Chinese coastguard vessels always sail near the islets, it was the first time an armed coastguard ship had entered the area, a Japanese coastguard spokeswoman told Reuters.
China responded to the allegations by saying they did not do wrong as the vessels were just carrying pieces of equipment.
"We have delivered our strong protest and requested [the Chinese coastguard] to stop the activities near the Senkaku Islands immediately," Takako Ito, a spokeswoman at Japan's Ministry of Foreign Affairs, said in an email.
"Japan will continue to act firmly and calmly, under the principle of resolutely defending our territorial land, sea and air," she added.
China and Japan have historically had a difficult relationship, though both countries have tried to improve that relationship in current years.
Last year in November, Chinese President Xi Jinping and Japanese Prime Minister Shinzo Abe met to discuss tensions over three of the disputed islands in September 2012.
China claims most of the energy-rich South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims