Three Chinese coastguard ships on Sunday remained near disputed waters around a set of islands controlled by Tokyo but claimed by both countries after entering the contested zone a day earlier, Japan's coastguard said.
Four Chinese coastguard vessels have stayed in the area in the past several days, with three of them sailing into the territorial waters shortly after 5:00 pm (0800 GMT) on Saturday, the Japanese coastguard said.
It was the latest in a series of incursions by Chinese government ships in recent months around the Senkaku islands, which Beijing claims as the Diaoyus.
The dispute has been cited as a potential flashpoint that may even lead to armed conflict between the Asian giants.
All the ships left the 12-nautical-mile band of waters by 11:00 pm, with three sailing into the contiguous zone on Sunday, the agency said.
Contiguous waters are defined as a maritime zone adjacent to a country's territorial sea where a coastal state may exercise controls necessary to prevent and punish infringement of its laws.
The Japanese coastguard also said its vessels spotted a small yacht believed to be of British registry raising the Chinese flag some 50 kilometres (31 miles) northwest of one of the islands and later entering territorial waters shortly after 5:00 pm Saturday.
Sailers of the yacht then threw a buoy with what the agency described as a red flag into the water and left the area, it said.
The incidents followed the first entry by coastguard ships since Beijing combined several agencies under the coastguard flag last month, a development that observers said would involve the arming of more crew.
Observers say the presence of a large number of official vessels, some of them armed, increases the likelihood of a confrontation since a minor slip could quickly escalate.
Chinese media have reported that the unified coastguard agency integrates marine surveillance, the existing coastguard -- which came under the police -- fisheries law enforcement and customs' anti-smuggling maritime police.
The disputed islands are located in rich fishing grounds and are believed to harbour vast mineral reserves below their seabed.