Drills involving U.S. Air Force F-15 fighter jets from the Kadena Air Base in Okinawa will be gradually incorporated into those of six Air Self-Defense Force bases from around February now that the two countries have effectively agreed on the cost burden, bilateral sources said Thursday.

The formal agreement may be concluded in mid-January, they said.

Joint drills by the two countries will be conducted in fiscal 2007, which starts April 1, two or three times at each six base, including Chitose, Hokkaido, and Hyakuri, Ibaraki Prefecture, further solidifying the bilateral military relationship, according to the sources.

While Washington initially wanted Japan to shoulder all of the costs, arguing that the transfer would ease Okinawa's U.S. base burden, it has accepted Tokyo's call for bearing part of the expenses as the plan will shore up interoperability, the sources said.

The two countries agreed in May to transfer part of the fighter jet drills at U.S. bases in Kadena, Misawa, Aomori Prefecture, and Iwakuni, Yamaguchi Prefecture, to Chitose, Misawa, Hyakuri, Komatsu, Ishikawa Prefecture, Tsuiki, Fukuoka Prefecture, and Nyutabaru, Miyazaki Prefecture.