U.S. Defense Secretary Robert Gates was to meet Tuesday with Japanese Foreign Minister Katsuya Okada on a visit to Tokyo where the Futenma base was expected to be a top topic.

"There really, as far as we're concerned, are no alternatives to the arrangement that was negotiated," Gates told reporters late Monday as he headed to Japan.

At issue is the fate of the air field, a major Marine hub, in the wake of Japanese Prime Minister Yukio Hatoyama's election last month.

U.S. and Japanese officials in 2006 agreed to shift 8,000 Marines to the U.S. territory of Guam and to move the Futenma base to another U.S. military location on Okinawa, Camp Schwab, where a new runway would be built.

But some members of Hatoyama's government want the Futenma base closed and the remaining U.S. troops moved out of Japan altogether. Okinawa residents have complained that the military bases cause too much noise and crime.

U.S. officials hope to resolve the issue by the time President Barack Obama arrives in Japan early next month.

Gates said he did not expect Hatoyama to renege on the agreement. The two men are scheduled to meet on Wednesday.

"This is an agreement between our countries, between our governments," Gates said. "And frankly I have every confidence that both sides will fulfill the commitments that they have made in this agreement."

If the base were forced to move, Gates suggested that the entire deal to relocate troops to Guam might fall through. "It's hard for me to believe that the Congress would support going forward in Guam without real progress with respect to the Futenma replacement facility," he said.

He said he had no problem with Hatoyama's decision to review the agreement, and cited "some flexibility" in terms of where, precisely, a new runway might be built at Camp Schwab. But ultimately, Gates said, the runway location is a matter for the local Okinawa government to decide with Tokyo.

The United States is Japan's key military ally, and an estimated 50,000 American troops are deployed there.

The issue of Japan withdrawing two of its naval ships from the Indian Ocean _ tankers that have been used as refueling pit stops for Afghanistan-bound allies _ will also be discussed over the next two days, Gates said.

However, it was unclear whether Gates thought he could reverse that decision.

"A number of countries benefit more from the refueling than the United States does," Gates said. "So I don't see the refueling as being a favor to the United States, but rather a contribution that the Japanese have made that is commensurate with its standing in the world as the second-wealthiest country and one of the great powers."

He added, "I'm sure we'll talk about it."