TOKYO — President Barack Obama said today his decision about how many troops to send to Afghanistan will come soon and he is bent on "getting this right."

In a joint news conference with Japanese Prime Minister Yukio Hatoyama, Obama rejected claims that his administration is dithering. The policy must protect America from terrorist networks, Obama said, while also making clear there is no "open-ended commitment" to Afghanistan.

Obama and Hatoyama pledged to renew their nations' alliance to keep pace with a fast-changing world.

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 noted Japan will no longer refuel ships that supply Afghanistan, but he promised aid for Afghan civilian needs such as schools, agriculture and police. He also vowed to cooperate with the United States on combating climate change and nuclear proliferation.

Obama arrived in Tokyo today, opening a weeklong trip to East Asia.

The two leaders are hoping to shore up relations with a nation that vows to be more assertive with the United States, even as Obama also weighs whether to send more troops to the Afghan war.

Obama's visit comes at a time of uncertainty in U.S.-Japan relations. Hatoyama said he would end Japan's Indian Ocean refueling mission that supports U.S.-led forces in Afghanistan and would review an agreement on relocating American troops in Japan that Washington thought was settled three years ago.

Obama also said the U.S. and Japan would work quickly to resolve a dispute over American military bases on Okinawa.

Hatoyama recalled he had campaigned on the issue of moving the U.S. base away from populated areas. He said the issue had to be settled quickly because delay would only cause the matter to fester.

Both men mentioned the stickiest issue in relations — the relocation of Marine Corps Air Station Futenma on the southern island of Okinawa — but offered no details. 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.

Weighing on Obama as he begins his trip is the pending decision on Afghan war strategy. 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."