Beijing has ploughed billions of dollars into modernising its military in recent years, a trend that has unnerved some governments in East Asia even though China claims its aims are peaceful.

China has two carriers currently in operation -- the Liaoning and Shandong -- while the Fujian is undergoing sea trials.

The Soviet-built Liaoning is China's oldest aircraft carrier, commissioned in 2012. The Shandong entered service in 2019.

China's navy said on Friday that the cross-regional trials and training for the Fujian "is a normal part of the aircraft carrier's construction process".

It is "not directed at any specific target", Leng Guowei, a spokesman for the Chinese navy, said in a statement.

Japan's defence ministry said that it had identified three Chinese naval ships on Thursday afternoon, including the Fujian, advancing southwest in waters approximately 200 kilometres (124 miles) northwest of one of the disputed Senkaku Islands.

"Among these, the Fujian aircraft carrier was confirmed by the Japan Maritime Self-Defense Force for the first time," it said in a statement.

Last year, Chinese vessels sailed near the Japanese-administered Senkaku islands -- known as the Diaoyu in China -- a record 355 times, according to Tokyo.

Japan said in July that China's intensifying military activities could "seriously impact" its security, citing the first confirmed incursion by a Chinese military aircraft into its airspace last August in an annual threat assessment.

China also said that a coast guard fleet had "patrolled within the territorial waters of the Diaoyu Islands" on Friday.

"This was a rights-protection patrol lawfully carried out," the China Coast Guard said in a statement.

- Expanding its reach -

Analysts at Washington-based think tank CSIS have said the Fujian is expected to feature more advanced take-off systems, allowing the Chinese air force to deploy jets carrying larger payloads and more fuel.

China has stepped up a massive expansion of its naval forces in recent years as it seeks to expand its reach in the Pacific and challenge a US-led alliance.

Beijing said in June that its Liaoning and Shandong carrier formations conducted combat drills in the western Pacific Ocean, unsettling regional neighbours including Japan.

A Taiwanese security official also said that month Beijing had deployed its two aircraft carrier groups around the island in May.

The Chinese Communist Party has refused to rule out using force to seize control of Taiwan, a democratic, self-ruled island that China insists is part of its territory.

The US Department of Defense said in a December report that China numerically has the largest navy in the world, with a battle force of more than 370 ships and submarines.