A Chinese coastguard ship equipped with what appeared to be four gun turrets was sighted near disputed islands in the East China Sea, Japan's coastguard said on Wednesday.
Chinese coastguard vessels sail regularly near the islets, but a Japanese coastguard spokeswoman told Reuters it was the first time an armed Chinese coastguard ship had been spotted in the area. For its part, China said the vessels were carrying standard equipment and doing nothing wrong in Chinese waters.
"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.
She referred to the group of tiny, uninhabited islands by their Japanese name. They are know as the Diaoyu in China.
"Japan will continue to act firmly and calmly, under the principle of resolutely defending our territorial land, sea and air," she said.
Chinese Foreign Ministry spokesman Hong Lei reiterated China's stance that the islands have been Chinese territory since ancient times.
"Patrols by Chinese coastguard ships in the relevant seas are beyond reproach," Hong said at a daily news briefing. "The equipment on Chinese coast guard vessels is standard equipment and no different from international practice."
The vessel, first sighted on Tuesday afternoon, was sailing 29 km (18 miles) off one of the contested islands as of 9:00 a.m. (0000 GMT) on Wednesday, Japan's coastguard said.
Sino-Japanese ties have long been mired by the dispute over the Japanese-controlled islands.
Patrol ships and fighter jets from both sides regularly shadow each other near the islands, stoking concern that an accidental collision could trigger a clash.
Japan said in 2013 a Chinese navy vessel aimed fire control radar, normally used to aim weapons at a target, at a Japanese navy ship in the East China Sea, prompting Japan to protest.
China also 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.