Four Chinese maritime surveillance ships have been spotted in territorial waters around disputed Tokyo-controlled islands, Japan's coastguard has said.
The ships entered Japan's 12-nautical-mile territorial waters around the East China Sea islands at around 02:00GMT on Sunday and sailed out to the contiguous zone after a few hours, the coastguard said.
It said the Chinese ships were not the same as those which spent several hours on Thursday in territorial waters around the islands claimed by both Tokyo and Beijing. That incursion provoked a strong protest by Japan.
The so-called contiguous zone is an area that extends a further 12 nautical miles beyond the territorial waters.
Tensions have risen in recent months over the islands, known as Senkaku in Japan and Diaoyu in China, which lie in rich fishing grounds. The seabed in the area is also believed to harbour mineral reserves.
China's state news agency Xinhua, citing the country's State Oceanic Administration, said the vessels were engaged in "routine patrols and law enforcement activities" in the waters.
It said the Chinese fleet told the Japanese coastguard ships to "immediately leave China's territory", while monitoring them "for the purpose of collecting evidence of infringement on China's sovereignty".
Tokyo and Beijing are reportedly preparing for talks on the island row which has hit multi-billion dollar trade ties between Asia's two largest economies.
Japan claims it had taken ownership of the islands already in 1895. China, meanwhile, claims the disputed islands have been part of its territory since "ancient times" and that Japan illegally took the islands.