The United States has basically agreed to return part of the airspace over Yokota Air Base in Tokyo as part of the realignment of U.S. military forces in Japan, informed sources said Saturday.

The basic agreement is expected to alleviate the overcrowding caused by the 470 commercial flights that must take detours around the so-called "Yokota RAPCON (Radar Approach Control)" area each day.

The Yokota RAPCON covers the airspace above Tokyo and eight prefectures -- Tochigi, Gunma, Saitama, Kanagawa, Yamanashi, Niigata, Nagano and Shizuoka. The military airspace is 7,000 meters high at its northern part and 3,700 to 5,500 meters in its southern part near Tokyo.

Flights bound for western regions, such as Chugoku and Kyushu, have to ascend to avoid entering the banned airspace, while flights originating from those regions must make a detour south of Yokota, according to the transport ministry.

The bilateral deal will likely allow Japan to use the highest part of the airspace, set new flight routes and shorten flight times, the sources said.

The details of the agreement will be hammered out before the U.S. military realignment plan is finalized at the end of the month, the sources said.

The agreement, however, will effectively shelve Japan's request for the complete return of the airspace, which it has been seeking since the 1980s.

The rising air traffic over the Kanto plane is believed to have been a major factor behind the decision to press for a partial return of Yokota RAPCON, the sources said.

In 2009, Haneda airport in Tokyo will inaugurate its fourth runway, while Narita International Airport in Chiba Prefecture will have one of its two runways extended to 2,500 meters from the current 2,180 meters in fiscal 2009.