Japan is seeking to buy four Boeing KC-46A aerial refueling tankers from the United States through the U.S. Foreign Military Sales program.

The proposed sale, which has been approved by the U.S. State Department, would be worth $1.9 billion.

"The proposed sale increases Japan's capability to participate in Pacific region security operations and improves Japan's national security posture as a key U.S. ally," The U.S. Defense Security Cooperation Agency said in its notification to Congress. "This proposed sale will provide Japan a needed capability to a close ally and support U.S. security interests in the region."

The agency, which manages the FMS program, said the proposed sales package to Japan would include related equipment, training, and support.

The new Boeing KC-46 Pegasus will be introduced into service in 2018 with the U.S. Air Force. It will have a 65,000-pound payload, a maximum transfer fuel payload of 207,672 pounds, a cruise speed of 530 miles per hour, and a range of 6,385 miles.

Each aircraft will be delivered to Japan with GPS capability and defensive systems installed and will include Raytheon's ALR-69A radar warning receiver, Raytheon's miniaturized airborne GPS receiver, a large aircraft infrared counter-measures system, and other specialized gear.

The principal contractors on the sale would be Boeing, supported by Raytheon and Northrop Grumman.