Tokyo: A Chinese naval ship sailed into waters surrounding disputed East China Sea islands for the first time early Thursday, prompting Tokyo to summon the Chinese ambassador to protest, the Japanese government said.
Russian naval ships were also seen in the area around the same time, according to local media.
"Around 00.50 am (1530 GMT Wednesday), a Chinese naval vessel entered our nation's contiguous waters surrounding the Senkaku Islands," the Japanese foreign ministry said in a statement.
Japan administers the uninhabited isles under that name while China also claims them and calls them the Diaoyu islands.
Relations between Japan and China deteriorated in 2012 when Tokyo "nationalised" some of the islands.
Since then, the two largest Asian economies have taken gradual steps to mend fences but relations remain tense.
Japanese Vice Foreign Minister Akitaka Saiki summoned Chinese ambassador Cheng Yonghua around 2.00 am to lodge a protest.
Saiki "expressed grave concerns and protested, while demanding the ship immediately leave our nation's contiguous zone," the ministry statement said.
Japanese diplomats and defence officials could not be reached immediately for further comment.
During his meeting with Saiki, Cheng claimed the Chinese frigate was allowed to sail in the waters, Kyodo News said, citing an unnamed source.
The frigate left the zone at about 3.10 am, according to major Japanese media, including Kyodo and national broadcaster NHK.
Chinese coast guard vessels routinely travel around the disputed islands, but this was reportedly the first time a Chinese navy ship has been spotted.
Three Russian military vessels were also seen in the waters around the disputed islands around the same time, Japanese media said.
The Russian ships entered the area around 9:50 pm Wednesday and left around 3.05 am Thursday, Jiji Press said, adding that Russian naval ships have entered the waters before.