Japan must develop an offensive military capability to defend itself against China while deepening its economic and military ties with the U.S.

Japanese defense experts agreed any successful defense of Japan means the country must have the means to attack the China mainland if needed.

They said Japan must bolster its offensive and counterstrike capabilities with an emphasis on precision long-range missiles.

The China threat to Japan's security is a "very severe reality" that requires "Japan (to) acquire more offensive capability," according to Masafuma Iida, head of the America, Europe and Russia division at the Japanese National Institute for Defense Studies.

Iida said more cooperation with the U.S. would start with "a common objective policy to Chinese assertiveness." This will include more joint military exercises and broader intelligence, reconnaissance and surveillance of China and its activities in the East China Sea.

Strengthening the military capabilities of the Quadrilateral Security Dialogue, a grouping of Japan, India, the U.S. and Australia and better known as "Quad," is required, Rumi Aoyama, a professor at the graduate school of Asia-Pacific Studies at Waseda University, said.

"Japan faces the need to defend its territory" against an expansionist China, Aoyama said. She called for more cooperation on defense among Quad.

Japan has already begun developing long-range missiles capable of precision strikes against China.