WASHINGTON (AP) _ The Pentagon plans to test part of its missile defense program Saturday by trying to shoot a mock warhead out of the sky with an interceptor rocket, military officials said.

The test had been scheduled for Oct. 24, but was delayed because of technical problems. U.S. officials say the test is designed to stay within the limits of the Anti-Ballistic Missile treaty that governs missile defenses.

President Bush has said repeatedly that the United States needs an effective defense against long-range ballistic missiles and that the ABM treaty must not be allowed to stand in the way. Administration officials have said the Sept. 11 terrorist attacks, while not involving missiles, show the country is vulnerable to unconventional surprise attack that one day could come from missiles.

For now, however, the administration is adhering to the treaty while it attempts to persuade Russia to set it aside.

Saturday's test, in which a missile interceptor is fired from the Kwajalein Atoll in the Marshall Islands to chase down an intercontinental-range missile carrying a mock nuclear warhead, will be the first since July. That test was successful, although the previous one in July 2000 failed.

The missile intercept tests cost about $100 million each.

Bush has said that unless he gets an arrangement with Russia that accommodates his missile defense program, the United States will withdraw from the treaty, which it is permitted to do with six months' notice. Bush and Russian President Vladimir Putin failed to reach an agreement during Putin's visit to the United States this month.

Vocal supporters of missile defense have urged the administration to declare the treaty invalid _ since one of its two original signatories, the Soviet Union, no longer exists _ and move ahead with unlimited testing.