Speaking to reporters after a meeting with his Philippine counterpart in Manila, Kerry brushed aside misgivings about Japan's military spending as he sought to allay concerns over its security implications in the region.
"This is not a sudden response to something or anything that anybody should be upset about," Kerry said.
"Japan has the ability to play a more modern, engaged role. This is something we have been working on, and they have been planning for some period of time," he added.
"We are talking about constructive efforts within internationally accepted frameworks for peaceful and appropriate purposes," he said, citing Japanese involvement in humanitarian missions.
He said while it had not yet been decided what role a better-armed Japan would play, the boost to its military assets would give that country "the ability to participate more" in maintaining regional stability.
"We welcome that in the context of our strategic thinking," Kerry said.
Japan today announced it would spend about USD 240 billion to buy stealth fighters, drones and submarines to beef up its defence of far-flung islands amid a territorial row with China.
Kerry said Japanese and US foreign and defence ministers had previously laid out "a joint vision of Japanese-US cooperation in terms of security for the region and elsewhere.
