The United States government has formally notified Congress of a plan to seek the sale of 36 advanced attack helicopters to South Korea.

A deal under the Foreign Military Sales program, if sealed, would be worth up to US$3.6 billion.

In two separate notifications to Congress, the Defense Security Cooperation Agency at the Pentagon provided details of proposed sales of Boeing's AH-64D Apache Longbow Block III, or the Bell AH-1Z Cobra.

South Korea is in the process of selecting a preferential bidder for the purchase of 36 attack helicopters.

The two U.S. firms are in competition that also reportedly involves the Turkish Aerospace Industry T-129B.

Seoul is scheduled to choose the winner as early as in November, defense sources said.

The agency said an estimated cost for the supply of 36 Apache choppers is $3.6 billion as it includes fuselages, associated equipment, parts, training and logistical support.

It put an estimated cost for 36 Cobra helicopters at $2.6 billion.

Company officials, however, stressed the estimated costs are not fixed.

"The price in the notification to Congress is a maximum one in a sense, since it covers almost all equipment and services. It can go down in accordance with actual negotiations on the terms of sales," a Boeing official told Yonhap News Agency, requesting anonymity.

Meanwhile, the agency emphasized a need to strengthen cooperation in arms sales.

"It is vital to U.S. national interest to assist our Korean ally in developing and maintaining a strong and ready self-defense capability," it said. "The ROK (South Korea) intends to use new attack helicopters to more effectively secure its borders and littoral waters, a well as conduct counter-terrorism/counter-piracy operations."

Such a notice of a potential sale of key U.S. military goods is required by law and does not mean the sale has been concluded. (Yonhap News)