The test over the Pacific Ocean marked the first time that scientists had successfully collided one missile into another while both were travelling through space at high speed.

"This test is certainly a positive development for us," Defense Secretary William Cohen said Sunday in Manila, where he was travelling.

"It is a testament to the kind of technology we are capable of developing. And I think it is an important milestone in this effort to conduct research, development and testing of such a system," he said.

In Saturday's test, an unarmed Minuteman missile took off from Vandenberg Air Force Base, California shortly after 7 p.m. Pacific time (0200 GMT), Pentagon spokesman Lieutenant Colonel Rick Lehner said.

Another device known as an Exoatmospheric Kill Vehicle (EKV) was launched from Kwajalein Atoll in the Marshall Islands, about 6,800 kilometers (4,300 miles) west of California.

The kill vehicle built by Raytheon Corp. is a 1.4-meter (55-inch), 54-kilo (121-pound) bundle of sensors on a booster rocket, but the relatively small device successfully intercepted the missile at 7:32 p.m. (0232 GMT) some 225 kilometers (140 miles) over the Pacific, Lehner said.

"It was a successful intercept," he said.

But critics say the test is hardly conclusive proof that such a system will work. The target missile carried a beacon that guided it into the range of the EKV. Booster rockets to carry the EKV, special radar systems to track incoming missiles and a command network to control the system are all still under development and have not been tested.

Although Washington is developing the program, deployment of the system is banned under the 1972 Anti-Ballistic Missile treaty signed with Russia.

The treaty, a cornerstone of the Cold War nuclear deterrence policy, generally bars the United States and Russia from building or deploying defense systems which could shoot down incoming ballistic missiles.

Washington and Moscow embarked upon negotiations at the beginning of September to reopen the treaty and allow for defense progams, but they have failed to make inroads as Russia remains steadfast in its opposition to any modification of the text.

Pursuing the new system could also irritate relations with China, as proponents have claimed the defense is needed because Beijing supplies missile technology to other countries, including North Korea.

The missile defense system is designed to shoot down a small missile attack by countries Washington considers rogue states or by terrorists. It would not be a defence against a barrage of missiles launched by a major power, such as Russia or China.

Nineteen more tests are scheduled between now and 2005, but the Pentagon has a major review of its programs due in June, so decisions on the system's future will start coming after only two more tests.

The White House is expected to decide on when to deploy the system after the Pentagon finishes its review.

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