The US troops in South Korea will complete the upgrade of its Apache attack helicopter fleets late this year to help strengthen its war capability, officials said Wednesday.

A squadron of 21 Apache AH-64As left for the United States Tuesday to be converted into an advanced version of the Apache -- the AH-64D Longbow, spokeswoman for the US Troops Korea (USFK) Ha Haeng-ja was quoted by South Korean Yonhap News Agency as reporting.

"After a nine-month transition period, the improved versions will return to South Korea," she said.

The United States has already upgraded two of its three Apache helicopter squadrons based in South Korea, Ha said.

Longbows are the US military's latest generation of attack helicopters and were first used extensively in battle during the Iraq campaign last year. The Longbow has enhanced versatility, an integrated weapon system and better communication systems.

The conversion of the AH-64A to the AH-64D Longbow consists of improvements in three general areas -- the improvement of the airframe, the addition of a fire control radar target acquisition system and the attachment of fire-and-forget Longbow Radio Frequency Hellfire missiles.

The missiles can find and hit targets in obscured conditions, precluding the use of Hellfire laser-guided missiles.

The United States currently keeps 37,000 troops in South Korea.