WASHINGTON (AP) _ U.S. intelligence officials told lawmakers Wednesday they were caught unawares by North Korea's test-firing of a three-stage rocket.

``The fact that they had a third-stage capability was not predicted by the intelligence community, and they are doing a reassessment,'' Sen. Chuck Robb, D-Va., a member of the Senate Armed Services Committee, said after a classified briefing by CIA Director George Tenet and Army Lt. Gen. Patrick Hughes, head of the Defense Intelligence Agency.

U.S. intelligence has concluded that the launch, though ostensibly intended to put a satellite into orbit, demonstrates continued North Korean efforts to develop intercontinental-range missiles.

Sen. Bob Smith, R-N.H., who also attended the briefing, said, ``The drift of the concern is they're more advanced than we thought they were. The performance of this thing came as a surprise. This is a serious matter that has to be addressed.''

The briefing was classified at the code-word level, meaning Senate staffers were not allowed.

The Aug. 31 North Korean rocket test shocked the world, not only because of the rocket's multistage capability but also because the flight path flew over Japan. Although the rocket's third stage failed to place a satellite into orbit, as the North Koreans were attempting to do, the rocket flew farther than North Korea's rocket program was thought capable of.

U.S. intelligence satellites and ground- and sea-based monitoring systems closely tracked the rocket flight, concluding the solid-fueled third stage may have achieved a range of between 2,408 and 3,720 miles, the Pentagon said.

Sen. Joseph Lieberman, D-Conn., another member of the Armed Services Committee, said one worry is that the North Koreans may try to develop a missile launch capability from boats that could be positioned close to U.S. territory.

An American intelligence official who spoke on condition of anonymity said Wednesday that what some lawmakers consider an intelligence failure stemmed from the difficulty of gleaning information from a closed society such as North Korea.

``Everyone knows that North Korea is a very difficult intelligence target,'' the official said. ``It's difficult to get a good picture of what's going on there, given the nature of the regime.''

The CIA's chief national intelligence official for strategic and nuclear programs, Robert Walpole, disclosed in a speech to the Carnegie Endowment for International Peace last week that the agency is working on improving its missile-development warning capability.

The CIA had predicted North Korea would soon be testing the Taepo Dong 1 and 2 missiles, improved variants of the Soviet-era Scuds. The Taepo Dong missiles are two-stage systems that would employ unpowered separating warheads. North Korea tested a Taepo Dong 1 early this year and used that two-stage rocket as a base for the Aug. 31 three-stage launch.

``Although the launch of the Taepo Dong 1 as a missile was expected for some time, its use as a space launch vehicle with a third stage was not,'' Walpole said. ``The existence of the third stage concerns us. We had not anticipated it.''

Walpole, who was among officials briefing lawmakers Wednesday, said the CIA is trying to determine how light a payload the three-stage rocket would have to carry to be propelled over intercontinental distances. He said the CIA must improve its intelligence analysis beyond simply warning that a country has a missile program and might test a certain type in a given year.

``We need to include clearer language and more details about how we might and might not be able to warn about specific milestones in an ICBM development effort, judgments that will likely vary by country,'' Walpole said.

The CIA also sent senior officials, including Walpole, to Capitol Hill later Wednesday to brief the Senate Intelligence Committee.