The Chinese coastguard entered waters disputed with Japan for the first time Friday, straining an already tense situation as Tokyo mulled plans to establish a US Marines-style force to protect its islands.

Four vessels spent three hours in the territorial waters of a Tokyo-controlled archipelago, where they traded warnings with their Japanese counterparts.

The move -- by vessels whose crews were likely to be armed, according to academics -- marks an upping of the ante in the blistering row over ownership of the Senkakus, which Beijing claims and calls the Diaoyus.

It came the day Japan's defence ministry recommended establishing amphibious units and acquiring surveillance drones to protect outlying islands.

"To deploy units quickly in response to a situation, it is important... to have an amphibious function that is similar to (the) US Marines," capable of conducting landing operations on remote islands, it said.

The recommendation was part of an interim report approved by a high-level defence meeting on Friday, which said more hardware was needed to monitor distant islands.

"Our country has some 6,800 islands," and one of the world's largest areas of sea to patrol, Defence Minister Itsunori Onodera told reporters.

"So protecting the islands is an enormous task, especially if it only relies on manned aircraft as we do currently."

Asked if there was a specific target for the moves, Onodera demurred.

"We don't have a particular country in mind," he said.

The report will be reflected in Japan's long-term defence outline that is expected to be published towards the end of this year, but which a defence official said would not include any reference to a "pre-emptive strike" capability.

Japan's constitutionally-mandated pacifism is cherished, but East Asia's shifting power structures are testing received wisdom in Tokyo.

"We have this awareness that given changes in the security environment surrounding Japan, we have to discuss whether it is enough for us to depend on US forces in terms of capability to attack enemy territory," a defence official told reporters.

Japan and the US have a security treaty that requires Washington to come to Tokyo's defence if it is attacked.

The pact is part of a post-war settlement that left tens of thousands of American troops and much advanced weaponry in Japan, sometimes euphemistically referred to as an "unsinkable aircraft carrier".

A decades old row over the ownership of the Senkakus came to a head in September when Japan nationalised three of the islands. Since then, China has become increasingly active in the seas around them.

But the presence Friday in the islands' waters of four possibly-armed Chinese coastguard vessels could take the dispute up a notch.

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.

The official Xinhua news agency said the vessels had "patrolled the country's territorial waters".

Chinese media reported that the unified coastguard agency integrated 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 could 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.

Japan diplomat visits China amid strained ties
Tokyo (AFP) July 29, 2013 - Japan's vice foreign minister on Monday set off for a two-day visit to China, Tokyo said, the latest chapter in a bid by Prime Minister Shinzo Abe to hold high-level talks with Beijing.

The trip comes as territorial tensions and maritime skirmishes have all but frozen relations between the Asian giants.

Akitaka Saiki, the top bureaucrat in Japan's foreign ministry, was expected to meet with his Chinese counterpart Zhang Yesui and other senior Chinese officials, Jiji Press news agency and other Japanese media reported Monday.

"We coordinated the trip as Mr Saiki has hoped to visit China as soon as possible since he assumed his post" last month, a foreign ministry official told AFP.

On Saturday, Abe told reporters in Manila that he hoped to hold top-level talks with China.

One of his closest advisers also said at the weekend that the testy neighbours could soon hold a meeting.

"I think a summit will be held in the not-so-distant future," Isao Iijima was quoted by Japan's Kyodo news agency as saying.

The diplomatic overtures come on the back of a long-running dispute between Tokyo and Beijing over ownership of an East China Sea island chain.

Since the row flare anew last year, China has become increasingly active in the seas around the disputed archipelago, while it has cancelled diplomatic and cultural events involving Japan.

On Friday, the Chinese coastguard entered disputed waters surrounding the islets for the first time, straining an already tense situation as Tokyo mulled plans to establish a US Marines-style force to protect its claim on the islands.

Although Chinese government ships have been in and out of the waters for many months, this was the first time they have ventured there since Beijing combined several agencies under the coastguard flag last week.