US Defense Secretary Robert Gates on Tuesday rejected demands by Japan's new government to review a deal between the two countries about a controversial US airbase and the presence of American troops.
Gates made the comments on his plane en route to Tokyo, before he arrived as the first top US official to visit Japan since the new centre-left government took power here just over a month ago.
New premier Yukio Hatoyama pledged during campaigning to look again at the 2006 agreement struck by his conservative predecessor and the former Washington administration of George W. Bush.
That agreement was about the presence of 47,000 American troops in Japan and the planned relocation by 2014 of a major US airbase from an urban area to a coastal part of southern Okinawa island.
But Gates ruled out reopening talks on the deal, after Hatoyama earlier suggested the base may be moved off the island.
"We think we need to progress with the agreement that was negotiated," Gates said hours before he was due to meet Japan's Foreign Minister Katsuya Okada.
Gates said that although Obama's administration understood the new government's "desire to review certain policies," the agreement had been years in the making and other options for Okinawa had already been exhausted.
"We've looked at, over the years, at all these alternatives and they are either politically untenable or operationally unworkable," he said.
Gates' two-day visit is the first by a member of President Barack Obama's cabinet since Japan's new government took power to end decades of conservative rule and vowing less subservient relations with the United States.
Hatoyama's government, which in opposition criticised Japan abetting "American wars," has also announced it would end in January an Indian Ocean naval refuelling mission in support of the Afghanistan war effort.
The US military presence on Okinawa has long angered residents because of aircraft noise and the risk of accidents, while crimes committed by US service personnel have caused friction with the local community.
The US defense secretary was due to visit Seoul on Wednesday and Thursday before heading to Slovakia for a NATO meeting of defense ministers on Friday.
He said he expected to discuss in Japan and South Korea economic and other possible assistance in support of the NATO-led mission in Afghanistan.
Hatoyama's government has said it is considering new civilian aid as a substitute for the naval mission it plans to end next January that has supported NATO-led forces in Afghanistan since 2001.
Japan's Defence Minister Toshimi Kitazawa said Tuesday he would also study sending military personnel to help aid efforts in war-torn Afghanistan.
"If you look at the opinions of the international community, including Europe, I have come to worry whether civilian help alone is sufficient as an alternative," said Kitazawa, who was due to meet Gates on Wednesday.
"In what way can the Self Defense Forces participate in the efforts? The government has not been able to engage in substantial debate, but I personally wish to study the matter by soliciting ideas from many people."
Japan's armed forces are barred under the country's pacifist post-war constitution from offensive combat operations, but they have taken part in overseas humanitarian and military support missions.