Southeast Asian and Japanese leaders on Saturday will commit to strengthening "maritime security cooperation," according to a draft statement, at a summit in Tokyo focused on boosting economic ties but also guarding against China's growing regional assertiveness.

Beijing claims almost the entire South China Sea, a vital trade corridor, and its increasingly aggressive behavior in disputed areas has riled nations across the region as well as Washington.

Close U.S. ally Japan, which also has competing territorial claims with China, is upping its military spending and has boosted security cooperation in the Asia-Pacific region.