WASHINGTON — Facing a daunting array of Asian challenges, President Barack Obama left Thursday on his first major trip to the region, where a surging China and newly assertive Japan are chipping away at America's standing on diplomacy and trade.

Already the most traveled first-year president ever, Obama took off for Tokyo on an Asian journey that will add four countries — Japan, China, Singapore and South Korea — to the 16 he's already visited. The trip also will highlight a dramatically changing continent.

"One of my most important tasks is to continue to strengthen the relationship between the United States and Asia," Obama said in a pre-trip interview.

En route to Tokyo, Obama planned to rally troops at Elmendorf Air Force Base. That stop — Obama's first visit ever to Alaska — was a reminder of the momentous decision he's weighing on whether to order a major new troop buildup in Afghanistan.

At a war council meeting Wednesday, Obama rejected the four Afghan war options before him and asked for revisions that combines the best elements of the proposals, Defense Secretary Robert Gates said Thursday. The changes could alter the dynamic of both how many additional troops are sent to Afghanistan and their time in the war zone.

Obama is not expected to decide the Afghan troop question until after he returns from Asia late next week.

Also left behind for now was health care. Following last weekend's narrow House passage of an overhaul plan, Obama's top domestic priority could reach the Senate floor while he's half a world away.

Obama was arriving in Japan a day later than planned, his schedule scrambled by Tuesday's memorial for the shooting victims at Fort Hood, Texas. His stop in Singapore for the annual Asia-Pacific economic summit, originally scheduled for two days, was cut back to a mere 20 hours.

Awaiting Obama in Japan was a new prime minister, Yukio Hatoyama, who swept to power vowing a more equal partnership with Washington. He's also promised to halt Japan's refueling of U.S.-led forces in Afghanistan, review its basing agreement for 47,000 U.S. troops and explore the possibility of a new Asian trading block excluding the United States.

In a pre-trip talk with Japan's NHK network, Obama acknowledged Hatoyama's election as a "political earthquake" but played down any friction. "This is not a senior-versus-junior partnership," he said. "This is one of equals in which Japan has been an extraordinary contributor."

As evidence, the White House pointed to Japan's pledge of $5 billion to aid Afghan development.