Japan has lodged an official protest after three Chinese ships briefly entered waters close to disputed islands in the East China Sea.
Footage shows a Japanese boat surrounded by Chinese vessels near to the archipelago which Japan calls Senkaku and China has named Diaoyu.
Tokyo controls the islands after buying them from a wealthy Japanese family earlier this month.
That decision has inflamed already tense relations between the two neighbours and stoked anti-Japan protests across China.
Japan’s Chief Cabinet Secretary Osamu Fujimura told reporters on Monday that Tokyo has formally complained to Chinese officials.
“Through high-level diplomatic channels, we are strongly urging China to keep the surveillance ships away from our territorial waters and to leave the area immediately,” said Fujimura.
On Saturday, around 800 Japanese protesters marched through Tokyo, denouncing China as a “brute state”.
While a full-scale conflict is unlikely, factions within China are reported to have urged officials to prepare for military intervention.
The islands are also disputed by Taiwan.
Japan protests after Chinese ships sail close to disputed islands