The official, who recently spoke to reporters on condition of anonymity, also said the relationship of mutual trust between the two countries would suffer a setback if the existing deal on the realignment of U.S. forces in Japan, which covers the airfield issue, is not implemented.

It is the first time that a U.S. administration official has signaled that Washington would mull the idea of making adjustments to the location of the new facility that would take over the heliport functions of the U.S. Marines Corps' Futemma Air Station in Okinawa.

Defense Secretary Robert Gates, who will visit Japan on Tuesday and Wednesday, is expected to urge Tokyo to implement the existing accord during his meetings with Japanese leaders.

Okinawa Gov. Hirokazu Nakaima said recently that Okinawa would accept the relocation of the Futemma facility within the prefecture, provided that the new airfield is located as far away from the seashore as possible to ensure safety and reduce noise.

The senior Pentagon official said it would be a matter for Okinawa and Tokyo to settle if the Okinawa governor is eager to move the new facility a further 50 meters or so offshore than under the current plan.

But the official also said that if Japan proposes such an option, the United States would consider it within the bounds of the existing agreement on the realignment of U.S. forces in Japan.

Japanese Prime Minister Yukio Hatoyama, who took office in mid- September, has said the government will seek to move the heliport functions of the Futemma Air Station outside Okinawa, or even outside Japan.

That would contravene an accord reached in 2006 in which Japan and the United States agreed to transfer the air station to an area off the coast of Henoko in the city of Nago in the prefecture, near the U.S. Marine Corps' Camp Schwab, by 2014.

A source close to Japan-U.S. relations said moving the envisaged airfield by 50 meters or so would not legally require a fresh environmental impact assessment and so could be interpreted as staying within the framework agreed by the two countries.

Reiterating that the accord was the result of 15 years of negotiations, the Pentagon official said a full review of the Futemma transfer plan would spill over into other parts of the deal, creating a serious situation for bilateral ties.

On Japan's intention to end its refueling mission in the Indian Ocean in support of U.S.-led antiterrorism operations in and around Afghanistan, the official said that if Japan helps Afghanistan some other way, the United States would understand and support the move.

Japanese Parliamentary Defense Secretary Akihisa Nagashima recently notified the White House and Defense Department that Tokyo will halt the refueling mission after the legal mandate for the mission expires in January.

The refueling mission, in place since 2001, was briefly halted in November 2007 after a temporary law authorizing it expired. The operations resumed after a new law was enacted in January 2008 and were extended to January 2010 after an amendment last December.

As for North Korea, the official said the United States will continue to press the reclusive country to give up its nuclear weapons in a complete, verifiable and irreversible fashion.

Washington will never recognize Pyongyang as a nuclear weapons power, the official added.