Japan has summoned a China envoy to protest an unusually long visit by Chinese government ships near disputed waters in the East China Sea.
Tokyo's foreign ministry says the four ships entered the Japan-controlled territory on Wednesday. It says they had still not left 24 hours later.
Japanese Chief Cabinet Secretary Yoshihide Suga says acting ambassador Han Zhiqiang has been summoned to "sharply protest" the move.
"This intrusion into Japanese territorial waters is the longest since the Japanese government nationalized the Senkaku islands last September and is completely unacceptable," he said.
The decades-old dispute flared up late last year, after Tokyo purchased some of the contested islands from their private Japanese landowner.
Since then, Beijing has sent regular air and sea patrols near the islands, in what some say is an attempt to challenge Japan's control of the energy-rich, strategic area.
This has led to fears of an unintended military clash and has raised tensions considerably between the populations of the two countries.
There is new evidence that deteriorating relations could further impact the significant trade between the world's second and third largest economies.
A study conducted by the official China Daily newspaper found a growing number of people in both countries are pessimistic about economic relations.
The study found over 40 percent of Japanese believe bilateral ties will experience a downturn both politically and economically. It said the poll in China revealed similar figures.
Overall, nine out of 10 respondents in both countries said they dislike the other nation. That is the worst level in almost nine years since the poll was first conducted.
The official Xinhua news agency said the biggest reason for the negative attitude is the dispute over the Diaoyu islands, which are known as Senkaku in Japan.