TOKYO - U.S. leaders expect Japan's new government to stick with existing agreements between the two nations, America's top defence official said Tuesday amid debate over whether to shutter a U.S. military air field on Okinawa. Japanese officials, however, signalled that they expect flexibility from the U.S.

Pentagon chief Robert Gates did not specifically mention the controversy over the Futenma base, a major U.S. Marines hub, during brief public remarks with Japanese Foreign Minister Katsuya Okada. But he made clear that the Obama administration would frown on any action by Tokyo to block a new runway at another base on Okinawa when Futenma is closed.

"As your government exercises its new responsibilities, I want you to know the United States stands with you," Gates told Okada at the start of their half-hour meeting in Tokyo. "And we are committed to advancing and implementing our agreed alliance transformation agenda."

Okada told Gates the administration of newly elected Japanese Prime Minister Yukio Hatoyama would seek a "deeper and sustainable" relationship with the U.S. "Of course, we have a lot of challenges," Okada added.

At issue is the fate of the air field following Hatoyama's election last month. Hatoyama told reporters Monday night, "We will choose an appropriate time to come to a conclusion after thoroughly reviewing the situation and process of the agreement." Hatoyama said he was well aware of the weight of the agreement between the two past governments. "But it is also important for both of us to be flexible."

Three years ago, U.S. and Japanese officials agreed to shift 8,000 Marines in Okinawa to the U.S. territory of Guam and to move the Futenma air field base to Camp Schwab, also on the southern Japanese island.

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

A Japanese foreign ministry official in charge of Japan-U.S. security alliance said Gates reiterated his stance on the Futenma base issue, saying the current plan is the only one that can be achieved and it should be implemented as soon as possible under the current agreement.

Okada responded that he is well aware of the importance of the agreement but asked Gates to understand the change of political environment in Japan. He said Japan also hoped to resolve the air station relocation issue as quickly as possible but asked the U.S. to understand difficulties in Okinawa. He did not mention any timeline to resolve the issue.

U.S. officials hope to resolve the issue by the time President Barack Obama arrives in Japan early next month. Gates will meet with Hatoyama on Wednesday.

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 told reporters aboard his military jet on the way to Japan.

He said that other, unidentified locations for the air field that were reviewed by the U.S. and Japanese governments were either politically or operationally impossible.

Gates 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.

Another issue, that of Japan withdrawing two of its naval ships from the Indian Ocean -- tankers that have been used as refuelling pit stops for Afghanistan-bound allies -- was also discussed.

Gates told Okada that Japan's naval refuelling mission in the Indian Ocean is a major support for the U.S. coalition forces, though whether to continue the mission is Japan's decision. Gates urged Japan to continue providing "strong" support for Afghanistan and Pakistan.

Okada repeated that Japan is not automatically extending the refuelling mission and that the government is currently considering several Afghan reconstruction projects in areas Japan can best use its resources. The ministry briefer, who spoke on condition of anonymity due to ministry policy, did not specify the projects that are under way.