Tokyo (CNN) -- U.S. Defense Secretary Robert Gates said Friday that the presence of American forces in Japan is critical to meet security challenges in Asia amid the tension in the Korean Peninsula.
"To deal with this century's critical challenges, critical component will remain the forward presence of U.S. military forces in Japan," Gates said during a speech at Keio University in Tokyo. "Without such a presence, North Korea military provocations could be more outrageous or worse. China might behave more assertively toward its neighbors."
The relationship between Japan and the United States has been strained in recent years over relocation of U.S. forces from Okinawa, a southern Japanese island where most of the American forces are based.
Gates' message has been to look beyond the single thorny issue of relocation to the larger issue of security in the region.
He pointed out that North Korea has grown more "lethal" and "destabilizing." He also said China's support is necessary to defuse the tension on the Korean Peninsula.
Gates also urged Japan to take more active roles in security issues in the region.
"By showing more willingness to send self defence force abroad under international auspices consistent to your constitution, Japan is taking right places alongside with other great democracies."
Gates called China's military modernization as "opaque" and is a "source of concern to its neighbors."
He also pointed out that Chinese military in cyber and anti-satellite warfare could pose a challenge to the U.S. operation in the Pacific. The U.S. spots some "disconnect" between China's government and military, he said.
Gates, who flew from Beijing on Wednesday, left for Seoul immediately after the speech.