U.S. intelligence experts failed to anticipate North Korea's ability to launch a three-stage rocket last month and now suspect the secretive communist state may have moved closer to developing a missile capable of hitting the United States, according to a senior CIA official.

The North Korean rocket, fired Aug. 31 in an arc over Japan, broke up somewhere over the Pacific Ocean without reaching orbit. U.S. officials first depicted the launch as a test of the two-stage, medium-range Taepo Dong I missile. But two weeks later, they reported the rocket had three stages and carried a satellite that was destroyed in flight.

"Although the launch of the Taepo Dong I as a missile was expected for some time, its use as a space launch vehicle with a third stage was not," said Robert D. Walpole, the CIA's senior intelligence officer for strategic programs. "The existence of the third stage concerns us. We had not anticipated it."

Walpole's remarks were included in a speech delivered a week ago and widely distributed yesterday. The speech generally defended the intelligence community's efforts to assess foreign missile developments. But the North Korean case became a focus of a Senate hearing yesterday in which members of a congressionally mandated panel cautioned against the belief that the threat of a potential attack was a long time away.

"I think {the North Korean launch} reinforces the point that we cannot expect that we're going to know everything that's going on in the world," said Donald Rumsfeld, the former defense secretary and White House chief of staff, who chaired the commission.

The question of how well U.S. intelligence analysts can assess missile development abroad is critical to a heated political debate here over how fast to proceed with work on anti-missile defenses. The United States lacks any system for shooting down intercontinental missiles, and Republican lawmakers have pushed for greater spending and a firm decision to build a national shield. The Clinton administration has an anti-missile system in development, but has resisted a deployment decision, saying the technology remains untested and the threat of attack still some years away.

U.S. intelligence agencies have maintained that a long-range missile threat from potential Third World adversaries is unlikely to emerge before 2010, except possibly from North Korea, and would likely be detected well in advance. The Rumsfeld commission, which included scientific specialists and former government and military officials, reported in July that the threat from Iran and Iraq as well as North Korea was more imminent and said an attack could come with little or no warning.

Attempting to clarify the agency's current views, Walpole in his speech said more than any intelligence official had previously in public about the government's missile threat estimates. He said there was more agreement than divergence between the government's reporting and Rumsfeld's group, particularly on the threat from North Korea.

He described North Korea as having the most advanced missile development program among hostile states, significantly ahead of Iran and Iraq. He said analysts are still trying to figure out why the North Korean launch failed and its implications for U.S. security. But he noted that a three-stage configuration, with a light enough payload, could well give North Korea the ability to send warheads across the Pacific.

"In particular, the community is assessing how small a payload would have to be for this system to fly to something on the order of an ICBM range," Walpole said in his speech to the Carnegie Endowment for International Peace.

Walpole said the U.S. intelligence community still considers it "unlikely" that a Third World country other than North Korea will develop a missile "capable of reaching any part of the United States over the next decade." He also played down the possibility that Russia or China -- which he said now has 20 CSS-4 long-range missiles -- would sell missiles or production facilities to assist another country. But he expressed concern about North Korea doing so as its program advances.

He said intelligence agencies could provide "five years' warning" of any indigenous long-range missile development by a potentially hostile power. But he acknowledged there might be little warning if the missiles were bought or if they were adapted for launch from sea.