TOKYO - Japan needs amphibious units and surveillance drones to protect its outlying islands, the defence ministry said Friday, as possibly-armed Chinese coastguards plied the waters near a disputed archipelago.

In a paper that had been widely reported on Thursday, the ministry said it was necessary to boost the strength and range of forces that could be used to protect Japan's far-flung territories.

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

The interim report, which was approved by a high-level defence meeting on Friday, also advocates the introduction of a drone reconnaissance fleet that could be used to monitor distant islands. The report will be reflected in Japan's long-term defence outline that is expected to be published towards the end of this year.

The paper also stresses "the need to boost a comprehensive capability of containment" as part of anti-ballistic missile measures against North Korea. The phraseology reflects an on-going debate among politicians about the need to re-interpret aspects of the pacifist country's military stance, defence officials told reporters, and steered clear of any mention of "first strike". "We are not talking about pre-emptive attack. That's not good," a defence official said.

"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," he told reporters. Japan and the US have a security treaty that binds Washington to coming 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 a lot of hardware in Japan, sometimes euphemistically referred to as an "unsinkable aircraft carrier". Experts say North Korea, through the UN-banned tests of ballistic missile technologies and nuclear weapons, has improved its offensive capabilities.

China has become increasingly active in the seas surrounding Japan, including the waters near the Tokyo-administered Senkaku islands in the East China Sea, which Beijing claims as the Diaoyu islands.

The row over their ownership, which began decades ago, erupted last September when Japan nationalised three of the islands. It took another turn on Wednesday when possibly-armed Chinese coastguard vessels sailed through nearby waters for the first time.

Later in the day Tokyo scrambled fighters to shadow a Chinese reconnaissance aircraft that flew in international airspace in between two Okinawan islands.

It was the first time Beijing had sent a military plane through the gap and out to the Pacific Ocean, Tokyo officials said, adding they saw it as a sign of "China's ever-growing maritime advance".

Four Chinese coastguard vessels were back in the waters off the islands on Friday, their Japanese opposite number said.

Abe's administration decided to review the current long-term defence outline which was drafted by the now-opposition Democratic Party of Japan in 2010.

The hawkish premier this year boosted Japan's defence budget for the first time in over a decade against the backdrop of growing concerns among many countries in the region about China.