By Jim Wolf

WASHINGTON (Reuters) - The government may let other companies compete for upkeep of a key missile-defense program run by Boeing Co (BA.N: Quote, Profile, Research) for the past 10 years, the head of the U.S. Missile Defense Agency said on Monday.

The agency said on March 21 it had determined Boeing was the only qualified source to build out the ground-based mid-course (GMD) system, including tying in 10 proposed two-stage interceptor missiles to be placed in Poland.

"But we are still entertaining the idea of competing the maintenance and sustainment," Air Force Lt. Gen. Henry Obering told reporters at an annual conference on ballistic missile defense. "We need to take a look at what could be the most cost effective."

The GMD is designed to shoot down enemy ballistic missiles in the middle of their flight paths while they cruise through space. It is the Pentagon's costliest missile-defense program, accounting for about $2.3 billion in fiscal 2008, which began Oct. 1.

The GMD is the hub of a layered shield that also includes missile interceptors on ships, sensors in space and a potential airborne laser aboard a modified Boeing 747 aircraft.

Work on the GMD has brought Boeing about $16 billion since 1998, when it was awarded contracts for system development and sustainment.

Obering declined to say when the agency would decide whether to invite rival bids.

Chicago-based Boeing, the Pentagon's No. 2 contractor by sales after Lockheed Martin Corp (LMT.N: Quote, Profile, Research), plans to seek to continue the GMD sustainment work, said Scott Fancher, Boeing's general manager for missile defense systems.