"I am pleased that today's test of our ballistic missile defense system appears to have been a success," Donald Rumseld, the US secretary for defence, said in a statement released after the missile test on Friday.

The Pentagon's Missile Defence Agency said that an interceptor missile fired from California has successfully collided with a missile launched from Alaska.

The successful interception of the warhead gives an important boost to a military program that has been widely criticized in the US for being too expensive and ineffective.

In the nine or 10 full-fledged missile intercept tests to date, only five have shot down target missiles.

The US spends over $10 billion a year to develop the missile program that is designed to shoot down limited numbers of long-range nuclear missiles fired from states like Iran or North Korea.

More challenging tests ahead

Rumsfeld said that the successful test of the interceptor missile was one step towards making the system fully operational and he said that more tests would be carried out.

"Successful tests such as these increase confidence in the approach to developing an initial missile defence capability," he said.

"While today's test was a success, the test program is by no means complete. Tests will continue, some of which will be successful and some will not. This was a challenging test, and the tests will become even more challenging in the period ahead."

High speed collision

In the final phase of Friday's test a refrigerator-sized "kill vehicle" separated from the interceptor.

Moving at 20,000 kph (18,000 mph) it struck a 1.2 meter (4 foot) mock warhead released by the other missile.

Both disintegrated more than 160 kilometers (100 miles) above the Earth. The interceptor's flight lasted 13 minutes.

This was the first exercise involving a mock warhead target since interceptor rockets failed to leave their silos during tests in December 2004 and February 2005.

It was also the first since the ground-based system, part of a layered shield that also includes sea and space-based components, was activated following ballistic missiles tests on July 4 and 5 by North Korea.

General Henry Obering, head of the US Missile Defence Agency, has said that the interceptor missile system would have worked against a US-bound North Korean missile if a decision had been made to shoot it down.

The US has spent more than $100 billion developing the missile interceptor system since 1983.