Liberal Democratic Party secretary-general Shigeru Ishiba's comments come amid tensions with China over Japan's wartime past and a territorial dispute.
Speaking at a seminar in Washington, Ishiba struck a conciliatory tone but also cautioned China against using "misguided nationalism" to direct domestic discontent away from the ruling communist party and toward other countries.
He said "adventurism" could have disastrous consequences. Ishiba described the US-Japan security alliance as a "public good" for the region to prevent conflicts and deter use of force that would change the "status quo."
"This can play a big role in establishing peace in the Asia-Pacific region," Ishiba said.
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The government of Prime Minister Shinzo Abe is considering re-interpreting the nation's pacifist constitution to ease restrictions on Japan's military. China, which has unnerved its neighbors through its military buildup, has claimed this could herald a new era of Japanese militarism, which Tokyo rejects.
Ishiba indicated that said Japan may authorize its military to shoot down a missile headed for the US, or to protect a US ship. He said Japan should also make clear what would kind of military actions be prohibited, such as helping countries that don't request assistance or a disproportionate response to an attack.
Obama affirmed that the US would be obligated to defend its Japanese treaty ally in a confrontation with Beijing over a set of disputed islands. But he urged all sides to resolve the long-running dispute peacefully.