The U.S. State Department notified Congress of South Korea’s planned purchase of six Boeing P-8A patrol aircraft, in a deal worth an estimated $2.1 billion.

The new P-8A aircraft will be used to update the Republic of Korea’s existing fleet of P-3C Orion maritime surveillance aircraft, according to a U.S. State Department announcement.

Poseidon aircraft conduct anti-submarine warfare, anti-surface warfare, intelligence and reconnaissance missions. The aircraft field a variety of torpedo

For more than 25 years, the Republic of Korea has also relied on the P-3 to conduct maritime patrols. The commonalities between the aircraft used by both the U.S. military and their Korean counterparts have enabled both nations to coordinate coalition maritime operations, according to the State Department announcement approving the sale.

“The ROK is one of the closest allies in the [Indo-Pacific Command] Theater. The proposed sale will support U.S. foreign policy and national security objectives by enhancing Korea’s naval capabilities to provide national defense and significantly contribute to coalition operations,” read the Wednesday announcement.

Boeing is equipping each Korean P-8 aircraft with Multifunctional Information Distribution System Joint Tactical Radio Systems 5, AN/AAQ(V)1 Acoustic System, AN/ALE-47 Counter Measure Dispensing System and six AN/AAR-54 Missile Warning Sensors.

Boeing officials expect demand for the P-8A to increase, not only from countries such as South Korea which operate older P-3 airframes but form countries seeking to boost their maritime patrol capability, USNI News reported earlier this year.

During the Singapore Airshow, Boeing executives said the P-8 airframe benefits from technological advances made to the next generation of Boeing’s commercial 737 airframes. Boeing officials cited upgraded engines and improved fuel efficiency as one example of how 737 advances carried over to the P-8, USNI News reported.

The sale requires U.S. congressional approval.