TOKYO - Four Chinese ships spent more than 24 hours in what Japan sees as its territorial waters, prompting a Japanese protest to China on Thursday at a time when Japan has been signaling its desire for a summit.
Relations between the world's second- and third-largest economies have been strained for months, largely because of a dispute over a group of islands in the East China Sea.
But Japanese Prime Minister Shinzo Abe, fresh from an election victory last month that strengthened his hold on power, is keen to improve relations and has called for dialogue with China, although he has rejected any conditions on talks.
Chinese ships have previously been in waters near the uninhabited East China Sea islands that are controlled by Japan but claimed by both countries, but they have usually left after several hours.
"This is extremely regrettable and totally unacceptable,"Japan's chief cabinet secretary, Yoshihide Suga, told a news conference.
He said Japan had summoned an envoy from the Chinese embassy early on Thursday to protest "strongly" and demand the ships immediately leave.
The ships withdrew at around noon, Japan's coastguard said.
It was the longest stay by Chinese ships in waters near the islands since the dispute flared anew last September, after Japan bought several of the islands from a private owner, angering China.
China's Foreign Ministry did not immediately respond to a request for comment.
However, the country's State Oceanic Administration said in a statement posted on its website on Wednesday that four coastguard ships were conducting a patrol around the islands.
The ships had spotted Japanese ships "infringing China's sovereignty" and told them to leave, the Chinese agency said.
For months, aircraft and ships from both countries have played a cat-and-mouse game near the islands, known as the Senkaku in Japan and the Diaoyu in China, ratcheting up tension.
According to an annual poll sponsored by the official China Daily and the Japanese think-tank Genron NPO, 92.8 per cent of Chinese surveyed had a negative attitude toward Japan, 28 percentage points higher than in 2012 and the worst since 2005.
The survey, released this week, also showed that 90.1 per cent of Japanese had negative feelings toward China, up from 84.3 per cent last year. Both groups gave the island issue as the reason for their increasingly negative views.
Several of Abe's advisers have visited Beijing in recent months.
Japan's Vice Foreign Minister Akitaka Saiki met the Chinese foreign minister, Wang Yi, last week, though China has denied that talks on a possible summit are taking place.