President Obama's plans for reducing the U.S. nuclear arsenal and defeating Iran's missiles rely heavily on a new generation of anti-missile defenses, which last year he called "proven and effective."

His confidence in the heart of the system, a rocket-powered interceptor known as the SM-3, was particularly notable because as a senator and presidential candidate he had previously criticized anti-missile arms. But now, a new analysis being published by two anti-missile critics, at MIT and Cornell, casts doubt on the reliability of the new weapon.

Obama's announcement of his new anti-missile plan in September was based on the Pentagon's assessment that the SM-3, or Standard Missile 3, had intercepted 84 percent of incoming targets in tests. But a reexamination of results from 10 of those tests by Theodore A. Postol and George N. Lewis finds only one or two successful intercepts -- for a success rate of 10 to 20 percent.

Most of the approaching warheads, they say, would have been knocked off course but not destroyed. While that might work against a conventionally armed missile, a nuclear warhead might still detonate. At issue is whether the SM-3 needs to strike and destroy the warhead of a missile -- as the Pentagon says on its website.

"The system is highly fragile and brittle and will intercept warheads only by accident, if ever," said Postol, a former Pentagon science adviser.

The Pentagon's Missile Defense Agency strongly defended the SM-3's testing record and said that the analysis by Postol, an MIT physicist, and Lewis, a Cornell physicist, published in the May issue of Arms Control Today, was mistaken.

"The allegation is wrong," Richard Lehner, an agency spokesman, said Wednesday. He said the SM-3 is "attaining test scores that many other Defense Department programs aspire to attain." Even so, the Pentagon later admitted that four of the 10 analyzed flight tests carried no mock warheads at all.