U.S. Defense Secretary William Cohen on Tuesday night reiterated U.S. objections to a proposed 15-year time limit on the U.S. military's use of an airport to be built in Okinawa Prefecture.

"We would always continue to evaluate the situation according to the security needs at the time," Cohen said when asked about the proposed time limit during an interview on a flight to Seoul. He was flying from Thailand as part of a six-nation Asian tour.

"And so that is the way in which we intend to continue to analyze our presence in the region and how it is structured," he said. "It will be on a situational evaluation during the course of the future years, but that is the way in which the Japanese government and the U.S. government intend to handle it."

Cohen was referring to the 1996 Japan-U.S. Joint Declaration on Security, which stipulates that the two countries "will continue to consult closely on defense policies and military postures, including the U.S. force structure in Japan."

Cohen will fly to Tokyo on Friday for meetings with Japanese officials.

The airport, to be built in the northern Okinawa city of Nago, will take over the heliport functions of the U.S. Marine Corps' Futenma Air Station in Ginowan, central Okinawa, as part of a U.S. commitment to return the Futenma base to Japan.

Okinawa Prefecture is strongly pushing for the 15-year limit.

Japan and the United States agreed in 1996 on the return of the Futenma base within five to seven years on the condition that the heliport facilities be relocated within the prefecture.