Four Chinese coastguard ships were Wednesday near islands at the centre of a major territorial dispute, Japan's coastguard said, the first time that agency's vessels have entered the fray.
The ships, bearing the legend "China Coast Guard", were spotted by Japanese crews in the contiguous waters of the Senkaku islands, which Beijing claims as the Diaoyus.
Contiguous waters are a 12-nautical-mile band that extends beyond territorial waters. Under international rules, they are not the preserve of any one country, although the resident power has certain limited rights.
Vessels from China's Maritime Surveillance have frequently plied waters around the islands -- including territorial waters -- since a huge row over their ownership flared last year when Tokyo nationalised three of them.
Confrontations between Japan's coastguard and these ships have customarily been limited to an exchange of warnings, with each side telling the other to leave.
But this is the first time Beijing's coastguard has been seen in the area.
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.
And Arthur Ding, a Taipei-based researcher at the National Chengchi University, told AFP China's patrols in the South and East China Seas, where Beijing has separate territorial disputes with several nations, 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.
"Clampdowns on so-called illegal acts are likely to increase, with frictions with neighbouring countries expected to rise."
China's maritime capabilities have increased in recent years, and it has equipped its units with bigger vessels with longer ranges, capable of staying on patrol near disputed areas for longer periods, he added.
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.