TOKYO (AP) — 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 Defense 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-U.S. 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.

Chinese warships' advance into the Pacific has steadily increased, with the frequency of their passage off southwestern Japan tripling in the past three years, including in waters between Taiwan and its neighboring Japanese island of Yonaguni, the 534-page report said.

The report comes days after Japan demanded China stop flying its fighter jets abnormally close to Japanese intelligence-gathering aircraft, which it said was happening repeatedly and could cause a collision. Beijing, in return, accused Japan of flying near Chinese airspace for spying purposes.

Two earlier close encounters in June occurred over the Pacific Ocean, where Japan spotted two Chinese aircraft carriers operating together for the first time.

China's increasing dispatch of aircraft carriers in the Pacific underscores the country's attempt to advance its sea power in distant waters, the report said. It said China's frequent dispatch of bombers for long distance flights in the Pacific by more sophisticated flight routes and fleet organization is seen as Beijing's attempt to show off its presence around Japan and to further advance its operational capability.

The Defense Ministry noted two cases last year — a Chinese warplane's brief violation of Japanese airspace over waters off islands near Nagasaki and an aircraft carrier's entry into a zone just outside of Japan's territorial waters further southwest in the Nansei island chain.

With U.S. President Donald Trump focusing on the strengthening of the U.S. economy and security, Japan and other U.S. allies face expectations to play a greater role for peace and stability in the region, the report said.

North Korea poses "an increasingly serious and imminent threat" for Japan's security, the report said, noting the North's development of missiles carrying nuclear warheads into the Japanese territory and solid-fuel ICBM that can reach the U.S. mainland.

Russia maintains active military operations around Japan and violated the country's airspace in September, the report added, saying its increasing strategic cooperation with China has posed "strong concern" for Japan's security.