A top general says it could take two years to replace flawed missiles that are a key part of a Pentagon anti-missile defense system.

"There are quality control problems, and these have to do with the current set of missiles," Lt. Gen. Lester Lyles, director of the Pentagon's Ballistic Missile Defense Program Office, told a group of defense writers on Wednesday.The Theater High-Altitude Area Defense - or THAAD - missile program has suffered five successive test failures, and Pentagon officials are rethinking the program that already has cost $3.2 billion.

Lyles said results of reviews expected this month will determine whether a new THAAD test will be launched at the end of this year.

He said parts exist for two more missiles, all built at the same time as the ones already used. Other elements of the system - its radar, its battle management system, the missile launchers - have performed well in the tests and need not be scrapped, Lyles said.

"The missile is the only thing that's been a problem with the program," he said.

Recent tests by North Korea of its Taepo Dong 1 missile and by Iran of the Shahab 3 missile are adding to the worries of U.S. military commanders around the world, who want protection for their forces, the general said.

Despite the test failures, Defense Secretary William Cohen and the program's backers on Capitol Hill have urged its managers to keep trying, given the need for offering troops in the field more protection from longer-range missile attack.

THAAD is supposed to be an improvement on the Patriot missile system. It aims to strike an enemy missile at a high altitude - the so-called "hit a bullet with a bullet" concept. The next test, once scheduled for August, has been delayed until November or December.

The May test that failed was the eighth for the THAAD program and the fifth involving an actual missile flight and attempted intercept.