A prototype weapon for protecting all 50 U.S. states against ballistic missile attack passed a critical hurdle last night, acing its first intercept test by blasting a mock enemy warhead out of the sky, according to Pentagon officials.

The direct hit high over the central Pacific Ocean followed years of difficulties with earlier prototypes and moved the Clinton administration closer to approving construction of a national missile defense system. But the anti-missile plan still has many U.S. skeptics, and it threatens to disrupt relations with Russia and China.

In last night's test, an unarmed Minuteman rocket, simulating an enemy missile attack, took off from California at 10:02 p.m. Washington time and headed west. About 20 minutes later, an intercepting "exoatmospheric kill vehicle," mounted atop a booster rocket, was launched from the Marshall Islands about 4,300 miles away.

The Minuteman released a dummy warhead and a decoy balloon, designed to resemble the kind of intercontinental missile capabilities said by U.S. intelligence officials to be under development in North Korea and Iran. The interceptor's job was to pick out the warhead and slam into it.

Pentagon officials monitoring the test said the interceptor--an ungainly looking 55-inch, 121-pound assemblage of sensors, thrusters and guidance controls--found its target and obliterated it about 140 miles above the ocean.

"There was heavy cloud cover at lower levels, but the flash from the collision was very bright and unmistakable," said a military spokesman who watched a radar image.

Critics have argued that new arms control accords and tighter limits on technology transfer would be better ways of countering foreign missile proliferation than attempting to construct an expensive national shield that may not guarantee protection. They have faulted the test plan as unrealistic and too limited.

Only three flight tests are now scheduled before President Clinton faces a decision next summer over whether to field a national anti-missile system by 2005 at an estimated cost between now and then of $10.5 billion. Clinton has come under strong pressure from congressional Republicans to do so.

But significant technological hurdles remain. Last night's test focused just on whether the kill vehicle could locate and steer itself toward a small, fast-moving target in space. Other key components of the anti-missile shield--booster rockets for the kill vehicle, improved early warning and X-band radars to track incoming enemy warheads, special command and control networks to manage the system--are still in development and must be tested later.

Moreover, U.S. officials have only just entered talks with resistant Russian authorities to amend the 1972 Anti-Ballistic Missile Treaty to permit such a weapon.

The system being designed is far more modest than former president Ronald Reagan's "Star Wars" vision of a space-based umbrella that would make the United States impregnable to thousands of enemy missiles. Now the plan is for a limited defense--initially, 100 interceptors based at a single site in Alaska--to block whatever few missiles might be launched intentionally by rogue nations or accidentally by a major power such as Russia or China.

Arguing the urgent need for a national anti-missile system, proponents point to the spread of missile technology and the demonstrated willingness of Russia and China to provide technical assistance to potential U.S. enemies. North Korea's ability to test a three-stage, long-range missile in August 1998 caught U.S. intelligence officials by surprise and galvanized bipartisan support in Washington for moving ahead with a missile defense system.