Japan warned on Friday that China risked escalating tension with Taiwan with an increase in military exercises that appeared aimed in part at readying Beijing's forces for a possible invasion of the democratically governed island.

Japan's annual assessment of security threats, including those posed by neighbours China, North Korea and Russia, comes as Taiwan closely monitors Chinese air and sea exercises, including one with an aircraft carrier in the nearby Pacific.

"Because of that increase in military activity we cannot discount the possibility of heightened tensions," Japan said for the first time in its annual Defence White Paper.

The drills are the latest in a series including manoeuvres in the Taiwan Strait last year that a senior U.S. general said would be key to any invasion.

Tokyo's warning comes after NATO members meeting in Washington this week described China as a "decisive enabler" of Russia's war in Ukraine and said that Beijing continues to pose systemic challenges to Europe and to security. The remarks drew a sharp response from Beijing.

Japan worries that any conflict in Taiwan would spread to its territory.

With its westernmost island only 110 km (68 miles) from Taiwan, Japan hosts more than 50,000 American troops, hundreds of U.S. military aircraft, and an aircraft carrier strike group that Washington could deploy to defend Taiwan.

Japan's 548-page assessment also highlighted China's plans to double its nuclear arsenal to more than 1,000 warheads by 2030.

It mentioned attempts by neighbouring North Korea to boost its nuclear strike capability with surveillance satellites and new, more advanced missiles, some with sufficient range to strike the United States.

Japan also identified Russia as a concern, because of its military ties with China, and, more recently, with North Korea, which had begun supplying it with artillery shells and other ammunition for its forces fighting in Ukraine.

In June, Russian President Vladimir Putin signed a deal with North Korea's Kim Jong Un featuring a mutual defence pledge by which Moscow could offer military aid to Pyongyang.