Japan's government said it was the first time that a Chinese aircraft carrier, which was accompanied by two destroyers, entered its contiguous zone, an area up to 24 nautical miles from the Japanese coast.

"This incident is totally unacceptable from the perspective of the security environment of Japan and the region, and we have expressed our serious concerns to the Chinese side through diplomatic channels," Japanese government spokesman Hiroshi Moriya said.

China said the passage, which came less than a month after the first confirmed incursion into Japanese airspace by a Chinese surveillance aircraft, complied with international law.

The Liaoning carrier and two Luyang III-class missile destroyers were seen sailing southwards between the islands of Yonaguni and Iriomote on Tuesday and Wednesday, the Japanese defence ministry's joint staff said.

Yonaguni and Iriomote are near the uninhabited and disputed Senkaku islands in the East China Sea, known by Beijing as the Diaoyus, which have long been a source of friction between the two countries.

Taipei's government said earlier a Chinese naval formation led by the Liaoning sailed through waters northeast of self-ruled Taiwan on Wednesday and continued towards Yonaguni.

Contiguous waters are a 12-nautical-mile band that extends beyond territorial waters where a country can exert some control, according to international maritime law.

- Rattled -

China's growing economic and military clout in the region and its assertiveness in territorial disputes -- most recently with the Philippines -- have rattled the United States and its allies.

Analyst Yee Kuang Heng, of the University of Tokyo's Graduate School of Public Policy, said China "definitely expected this massive carrier to be detected" by Japan.

The ships may form part of a "strike group practising how they might deploy during a Taiwan contingency, and also pushing the envelope how far they can go needling Japan", Heng told AFP.

"Diplomatically, it may be a form of 'gunboat diplomacy' signalling to Japan to stay out of any potential Taiwan contingency, and a reminder that Yonaguni, which hosts important (Japanese military) bases, could find itself right on the frontlines," he said.

Tokyo has reported the presence of Chinese coastguard vessels, a naval ship, and a nuclear-powered submarine around the remote Senkaku islands.

Japan scrambled fighter jets in August after the first confirmed incursion by a Chinese military aircraft into its airspace, with Tokyo calling it a "serious violation" of its sovereignty.

It is ramping up its defence spending with US encouragement, moving to acquire counter-strike capabilities and easing rules on arms exports.

Tokyo is also providing funding and equipment such as patrol vessels to other countries in the region.

In July, Japan agreed on a deal with the Philippines allowing troop deployments on each other's soil.

Japan also scrambled fighter jets last week when Russian aircraft flew around the archipelago for the first time in five years.

The Tu-142 planes did not enter Japanese airspace but flew over an area subject to a territorial dispute between Japan and Russia, Tokyo said.

Russian and Chinese warships held joint drills in the Sea of Japan this month, part of a major naval exercise that President Vladimir Putin said was the largest of its kind for three decades.

The Japanese defence ministry said it had observed five Chinese naval ships entering the Sea of Japan and likely on their way to the joint manoeuvres.

nf-kaf-burs-stu/pbt