In a joint news conference with Japanese Prime Minister Yukio Hatoyama, Obama also said he was bent on "getting this right." He denied that his administration has been dithering and said that whatever policy change he announces must be aimed at protecting America from terrorist networks.
Opening a weeklong trip to Asia, Obama said the United States and Japan must "find ways to renew and refresh the alliance for the 21st century."
Hatoyama said the two leaders had agreed to spend the next year refining the nearly 50-year-old treaty that defines relations between the two World War II adversaries.
"Both Yukio and I were elected on the promise of change," Obama said. "But there should be no doubt that as we lead our nations in a new direction, our alliance will endure and our efforts will be focused so that it will be even stronger in meeting the challenges of the 21st century."
With Obama weighing a new strategy for Afghanistan, Hatoyama said Japan would end a refueling mission for the U.S. military. But he promised $5 billion in aid for Afghan civilian needs such as schools, agriculture and police.
The new Japanese leader also vowed to cooperate with the United States on combatting climate change and nuclear proliferation.
The trip came as the Obama administration was preparing to announce later Friday that the alleged mastermind of the Sept. 11 terrorist attacks, Khalid Sheikh Mohammed, would be moved from the Guantanamo Bay, Cuba, prison to New York for a civilian trial. Without confirming the details, Obama said Mohammed "will be subject to the most exacting demands of justice."
On Afghanistan, Obama said he was not waiting for any new information. Rather, he said, he wants to be sure he strikes the right balance before committing more U.S. troops and billions of dollars to the 8-year-old conflict.
On a stop at Elmendorf Air Force Base in Alaska on his way to Asia, Obama told a military audience he will commit more forces to Afghanistan only if it is vital to U.S. interests and receives public support.
"I will not risk your lives unless it is necessary to America's vital interests," Obama told the troops.
"And if it is necessary," he said, "the United States of America will have your back. We'll give you the strategy and the clear mission you deserve. We'll give you the equipment and support you need to get the job done. And that includes public support back home."
Obama and Hatoyama hope to shore up relations after the prime minister was elected on a platform of asserting greater Japanese independence in its dealings with the U.S.
Obama said the U.S. and Japan would work quickly to resolve a dispute over American military bases on Okinawa, one that Washington thought was resolved three years ago.
Hatoyama said the issue must be settled quickly because delay would only cause it to fester. The comment seemed to suggest he was moving closer to the U.S. position.
The prime minister's remarks were similar to those made by U.S. Defense Secretary Robert Gates during a recent trip to Tokyo. Hatoyama had said previously that he wanted to wait until possibly next year to resolve the issue of the Marine Corps Air Station Futenma.
Hatoyama has suggested moving the base off Okinawa altogether, while the U.S. wants to move it to a more remote location on the island, as part of a 2006 agreement on relocating 47,000 American troops in Japan.
Obama is on a mission to assure America's place in relations with a rapidly changing Asia that is reordering itself around China's surging economic and diplomatic clout. Obama's chief goal, the White House has said, is to demonstrate U.S. commitment to the region.
Obama also will travel to Singapore for meetings with Southeast Asian leaders, and then to China and South Korea. Many governments are keen to see a revitalized U.S. engagement in part to counterbalance China, and even a newly powerful Beijing says it welcomes a continuing U.S. role in the region.
Japan, long billed by Washington as the cornerstone of U.S. Asia policy, is caught up in these shifts. Hatoyama came to power calling for a more equal partnership with Washington and a more positive embrace of China, which will soon supplant Japan as the world's No. 2 economy.
In a pre-trip interview with Japan's NHK network, Obama sought to minimize any friction and likened the election of Hatoyama and his Democratic Party of Japan after nearly 50 years of rule by another party to a "political earthquake."
"I think that it is perfectly appropriate for the new government to want to re-examine how to move forward in a new environment," Obama said. "I don't think anybody expects that the U.S.-Japan relationship would be the same now as it was 50 years ago or 30 years ago or 20 years ago."
Associated Press White House Correspondent Jennifer Loven contributed to this report.