TEL AVIV Ñ Israel and the United States plan to test the Arrow-2 missile defense system against a target meant to simulate an advanced Scud D ballistic missile.

Israeli defense sources said the U.S. Defense Department and Israel's Defense Ministry intend to launch the Arrow-2 interceptor against a missile meant to simulate the Scud D. The sources said the test would take place by October off the coast of southern California in the United States under the Arrow System Improvement Program.

"We're talking about the most challenging test for the Arrow-2 in about a decade," a defense source said. "The test might very well fail, but we need to know how the Arrow matches up with the advanced missiles in Iran and Syria."

The Scud D is a medium-range missile developed by North Korea and can reach a distance of 700 kilometers. The missile has been supplied to Syria, which also began serial production over the last yeard.

On July 29, the Arrow-2 intercepted and destroyed the Scud B missile, with a range of 250 kilometers. Unlike the Scud B, the Scud D was deemed as about twice as fast and the warhead separates from the rest of the missile in the first stage of launch. The Scud B does not contain a separation process.

The forthcoming Arrow-2 flight, the sources said, would test the software upgrades of the interceptor and radar. They said the results of the test would help determine future goals and enhancements over the next two years.

The sources did not say where the United States obtained the Scud D-type missile. In January 2004, however, the United States removed the medium-range Scud C and other advanced missiles from Libya. One of those missiles was said to have a range of 800 kilometers.

Israeli officials acknowledge that despite the recent success of the Arrow-2, the system cannot yet defend against Iran's Shihab-3 missile, which has a range of more than 1,300 kilometers. They said the Shihab-3, which was flight-tested by Iran on Aug. 11, has a speed of Mach 10, considerably faster than the Arrow-2's speed of Mach 8.

The forthcoming Arrow-2 test was scheduled as one of a series of attempted interceptions of advanced Scud-based ballistic missiles. The sources said the tests were vital in the development of the Arrow-2 before full-rate production begins in the United States in 2005. Israel has already deployed two Arrow-2 batteries and plans to develop a third battery over the next two years.

The sources said the Scud D challenge was scheduled before the Shihab-3 launch and would help advance such U.S. missile defense projects as the Theater High Altitude Area Defense, PAC-3 and sea-based Aegis. The missile interception tests were meant to provide data on the affect of the sun on missiles with optical sensors.

"It is well known that Iran has had an active missile program for almost two decades, has been in the late stages of developing the Shihab-3 medium-range ballistic missile, and has been working on longer-range systems," a State Department spokesman said. "The United States has serious concerns about Iran's missile programs and views Iran's efforts to further develop its missile capabilities as a threat to the region and to United States interests."