Japan raised strong caution against China's rapid acceleration of military activity in extensive areas from around its southwestern coasts to the Pacific, describing the moves as the biggest strategic challenge. China's growing joint operations with Russia also pose serious security concerns to Japan, along with increasing tension around Taiwan and threats coming from North Korea, the Defence Ministry said in an annual military report submitted to Cabinet on Tuesday.

"The international society is in a new crisis era as it faces the biggest challenges since the end of World War II," the report said, citing significant changes to the global power balance while raising concern about an escalation of the China-US rivalry. The security threats are concentrated in the Indo-Pacific, where Japan is located, and could get worse in the future, report says.

Japan has accelerated its military buildup on southwestern islands in recent years, preparing to deploy long-distance cruise missiles, as it worries about a conflict in Taiwan, which China claims as its territory to be annexed by force if necessary. Taiwan launched 10-day annual live-fire military exercises last week intended to guard against Chinese threats to invade. Japan tested a short-range, surface-to-ship missile at home earlier last month.