US Secretary of Defense Robert Gates called on Friday on Asian states to work more closely together on security issues and not to depend only on the US to ensure regional stability.
Gates said that US moves to reduce its military presence in South Korea and Japan did not signal a declining US commitment to the region.
"The repositioning of our forces is really a reflection of the maturing of our alliance relationships here in Asia, and specifically with Japan and the Republic of Korea, and does not in any way represent a lessening of our commitment to either," he said.
In a speech and question and answer session, Gates said Japan should play a bigger role in international security.
"Japan has an opportunity -- and an obligation -- to take on a role that reflects its political, economic and military capacity," he said.
"That is why the US strongly supports Japan's becoming a permanent member of the United Nations Security Council."
Winding up a week-long visit to East Asia, Gates said the system of bilateral alliances that the US maintained in the region through the Cold War should evolve.
"To be honest about it, for many years the United States had a better relationship with a number of different Asian nations than they had with each other," he said.
"Now it seems to us there is a better opportunity for a closer US, Republic of Korea, Japan cooperation; US, Japanese, Australian cooperation; the involvement of India."
"And it seems to me this also reflects the growing importance of these countries in terms of their own security, and taking responsibility for their own security and not just depending on the United States to be the guarantor," he said.
US efforts to nudge Tokyo to take a bigger security role are controversial in Japan because of the pacificism enshrined in its post-war constitution, which restricts the Japanese military to a self-defense role.
A Japanese naval refueling mission in the Indian Ocean in support of the US-led "war on terror" has sparked intense political debate with the opposition blocking an extension of the mission when its mandate expired last week.
Gates suggested that the constitution could be interpreted in a way that would allow broader participation in international security operations without changing its pacifist character.
"That's a debate for the Japanese to have and for the rest of us to stay out of," he said.
Gates said the security challenges facing the world today are not easily dealt with by states acting on their own.
"The major challenges facing the region -- such as North Korea and nuclear proliferation -- cannot be overcome by one or even two countries, no matter how wealthy and powerful," he said.
Besides proliferation and terrorism, countries need to work together to maintain free and secure maritime routes, and prepare for responses to humanitarian disasters such as the 2004 tsunami or the spread of diseases.
He also reiterated US concerns about China's rapid military buildup and warned that "a lack of transparency carries the risk of misunderstanding and miscalculation, and naturally prompts others to take action as a hedge against uncertainty."
Gates began his trip to the region in Beijing, where he met with senior Chinese leaders before traveling on to South Korea.
"I do not see China as a strategic adversary. It is a competitor in some respects and a partner in others. While we candidly acknowledge our differences, it is important to strengthen communications and to engage the Chinese on all facets of our relationship to build mutual understanding and confidence," he said.