TOKYO -- Japan has decided to buy up to nine additional U.S.-made early warning aircraft, which are used to detect approaching foreign planes, sources have revealed.

The new E-2D aircraft, built by U.S. defense contractor Northrop Grumman, will cost the Japanese government 350 billion yen ($3.12 billion).

U.S. President Donald Trump, determined to reduce his country's trade deficit with Japan, has been pushing Tokyo to purchase more American defense equipment. He has repeatedly urged Japanese Prime Minister Shinzo Abe to pursue such deals.

Japan, for its part, aims to deflect Trump's criticism while stepping up monitoring around the Ryukyu Islands, which include Okinawa.

The latest plan comes on top of Japan's earlier decision in 2014 to buy four E-2Ds. The Ministry of Defense has been looking to upgrade its aging fleet of E-2C warning aircraft.

The additional purchase is to be incorporated into Japan's medium-term defense plan covering fiscal 2019 to 2023, to be worked out by the end of this year.

A fiscal 2019 budget request of 54.4 billion yen is to cover two E-2Ds, along with advance purchases of some components for the remaining seven.

The White House has already informed Congress about Japan's new purchase plan. According to Japanese officials, Abe explained the defense budget request including additional E-2D purchases when he met with Trump in late September.

The officials said the decision is mutually beneficial for the two countries, since it will fill a defense technology void for Japan and boost U.S. exports, including related equipment.

The annual U.S. trade deficit with Japan has reached about 7.5 trillion yen. And Tokyo is hoping to avoid additional tariffs on U.S.-bound auto exports, which could deal the Japanese economy a painful blow.