Chinese coastguard ships entered the territorial waters of Japanese-controlled islands at the centre of a bitter row, Japan's coastguard said on Friday, the first such incursion by the organisation.
Although Chinese government ships have been in and out of the waters for many months, this is the first time they have ventured there since Beijing combined several agencies under the coastguard flag this week, a development that observers said would involve the arming of more crew.
The move could further ramp up tensions around the Senkaku islands, which Beijing claims as the Diaoyus.
"We demanded they leave our territorial waters," an official at Japan's coastguard said.
The four vessels left the 12-nautical-mile band of waters around 1.30 pm (0430 GMT), approximately three hours after they arrived, he added.
Chinese media reported this week that a unified coastguard agency has gone into operation, integrating marine surveillance, the existing coastguard - which came under the police - fisheries law enforcement and Customs' anti-smuggling maritime police.
Chinese academics were reported as saying that the move would mean more armed ships in the region, while Arthur Ding, a Taipei-based researcher at the National Chengchi University, told AFP China's patrols were likely to become "more frequent and more forceful".
"As it is named the coastguard, (its ships) are likely to be authorised to carry light weapons so that they can enforce the law," he said.
Observers warn that the Senkakus are a potential flashpoint that may even lead to armed conflict.
They 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.
In one of the most serious incidents of the row so far, Japan said a Chinese battleship locked its weapons-targeting radar on one of its vessels. Beijing denied the charge, accusing Tokyo of hyping the "China threat".
While Japan's coastguard is a civilian organisation, it is well-equipped and well-funded, and some officers aboard the vessels are believed to carry sidearms.