China's Shandong carrier and four other vessels, including a missile destroyer on Monday, sailed inside the Japanese economic waters surrounding the remote Pacific atoll of Okinotori, Tokyo's defence ministry said.

Its fighter jets and helicopters conducted take-offs and landings there, the ministry said.

The fleet of five warships was also seen sailing on Saturday 550 kilometres (340 miles) southeast of Miyako Island near Taiwan, it added.

China's other operational aircraft carrier Liaoning and its fleet entered Japan's exclusive economic zone (EEZ) in the Pacific over the weekend, before exiting to conduct drills involving fighter jets, Tokyo previously said.

"This is the first time two Chinese aircraft carriers were spotted operating in the Pacific at the same time," a defence ministry spokesman told AFP on Tuesday.

"We believe the Chinese military's purpose is to improve its operational capability and ability to conduct operations in distant areas," he said.

China's use of naval and air assets to press its territorial claims has rattled the United States and its allies in the Asia-Pacific region.

Japanese and US defence officials say China wants to push the American military out of the so-called "first island chain" from Japan down through the Philippines.

Eventually, its strategy is to dominate areas west of the "second island chain" in the Pacific between Japan's remote Ogasawara Islands and the US territory of Guam, they say.

The Liaoning's recent cruise eastwards marked the first time the Japanese defence ministry has said a Chinese aircraft carrier had crossed the second island chain.

In September, the warship sailed between two Japanese islands near Taiwan and entered Japan's contiguous waters, an area up to 24 nautical miles from its coast.

At the time, Tokyo called that move "unacceptable" and expressed "serious concerns" to Beijing.

Under international law, a state has rights to the management of natural resources and other economic activities within its EEZ, which is within 200 nautical miles (370 kilometres) of its coastline.