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 security threats are concentrated in the Indo-Pacific, where Japan is located, and could get worse in the future, report says.