It would be difficult to reach agreement with the United States for a 15-year time limit to be set on the U.S. military's use of a new airport in northern Okinawa before construction begins, Foreign Minister Makiko Tanaka said Saturday.

Okinawa Gov. Keiichi Inamine and other prefectural officials are demanding that the time limit be set for the airport, which is to be built off northern Okinawa and shared by civilian air traffic and the U.S. military. It is designed to take over the heliport functions of Futenma Air Station in Ginowan, central Okinawa.

"I understand the governor's feelings. But if the construction is delayed (for the sake of setting a time limit), residents near Futenma will have to suffer longer and if it is moved up, it will conflict with the governor's plan to place the 15-year limit at an early time," she said during a news conference here.

"We must consider all of these issues together, while watching international developments and discussing the matter with the U.S.," Tanaka said.