SEOUL, South Korea — Less than 24 hours after diplomats announced a breakthrough pact to eliminate nuclear arms in North Korea, the isolated communist state threw cold water on the deal today, saying it would not abandon its weapons program until the United States gives it a light-water nuclear reactor.

North Korea's demand could be a deal breaker. Assistant Secretary of State Christopher Hill, the chief U.S. negotiator at the six-nation nuclear talks, made it clear at the opening of negotiations in Beijing last week that the idea of providing North Korea with a reactor before disarmament would be a "nonstarter."

No one is sure if the North can make a weapon small enough to mount on a warhead, or a missile that can deliver an accurate strike. North Korea has tested missiles that can reach Japan, China, South Korea and possibly parts of the United States, including the West Coast.

After four rounds of discussions spanning several years, the United States, North and South Korea, Japan, Russia and China said yesterday that they had agreed on a vaguely worded plan under which North Korea promised to dismantle its nuclear weapons in return for energy assistance, eventual U.S. and Japanese diplomatic recognition and a pledge by Washington that it would not attack the country.

The agreement also said the United States and other nations would discuss giving North Korea a light-water reactor for generating electricity, though it skirted the question of when. It said only that the possibility would be considered "at an appropriate time."

But the North Koreans made it clear today that they are in no mood to defer their reward.

In a blunt statement, North Korea's official KCNA news service warned that "the U.S. should not even dream of the issue of [North Korea's] dismantlement of its nuclear deterrent before providing" a light-water reactor, which it called "a physical guarantee for confidence-building."

Should the United States insist on the dismantling of North Korea's atomic weapons before the provision of light-water reactors, the statement said, "there will be no change in the nuclear issue."

Even though difficulties were expected to arise in the coming months as diplomats sought to iron out the details of yesterday's pact, the speed with which the Pyongyang regime punctured the celebratory mood was surprising.

Late yesterday, State Department spokesman Sean McCormack made a terse statement about the North Korean demand, telling reporters in New York, "This is not the agreement that they signed, and we'll give them some time to reflect."

In Washington, experts said that although North Korea's statement did not completely annul what was achieved in Beijing, it would not help the next round of talks, scheduled for November.

"I don't think it blows up the deal. What it does is underline how difficult the process remains," said Robert Einhorn, who was a top player in North Korean negotiations with the Clinton administration.

Einhorn pointed out that the document signed in Beijing was only the vaguest statement of shared principles. "This wasn't a big substantive reconciliation. This was an agreement to set aside a disagreement and move ahead with the talks."

Proliferation experts and intelligence reports indicate that North Korea has extracted enough material from a nuclear reactor it established at its Yongbyon complex to produce several nuclear weapons.

The U.S. is building a state-of-the-art missile interceptor system, partly in Alaska, for defense against hostile nations such as North Korea.

Before North Korea's announcement today, President Bush and others cautiously welcomed yesterday's development.

"They have said — in principle — that they will abandon their weapons programs," Bush said of North Korea. "And what we have said is, 'Great. That's a wonderful step forward.' But now we've got to verify whether that happens."

International Atomic Energy Agency Director-General Mohammed ElBaradei, whose organization would play a key role in verifying North Korea's disarmament, likewise hailed the announcement as "very welcomed news" but was cautious about the details.

"We'll see how this proposal will be translated into specific details," he said in Vienna, Austria, where the IAEA is headquartered.

Experts say the hardest part of the proposal would be getting North Korea to accept an intrusive presence of weapons inspectors to verify that its nuclear programs have been fully dismantled.

ElBaradei and others noted that it has been 12 years since there was a regular inspection program in North Korea. The country suspended its membership in the Nuclear Non-Proliferation Treaty in 1993, and put limitations on IAEA inspections even at its one designated nuclear facility at Yongbyon. It finally withdrew from the treaty in 2002 and kicked out inspectors.

Pierre Goldschmidt, a former deputy director-general of the IAEA who is now at the Carnegie Endowment for International Peace, said it would be "very important" for the agency to have "extensive rights to go anywhere they want."

Gary Schmitt, executive director of the neoconservative Center for the New American Century in Washington, was skeptical that rigorous inspections would be possible.

"You'll never find a perfect verification deal with North Korea. There's not enough inspectors in the world to make that happen," he said. "The question is whether you can [keep] them so busy keeping things hidden that they become operationally less effective. I have serious doubts that we'll get that kind of inspection regime."

Los Angeles Times staff writers Alissa J. Rubin in Vienna and Bruce Wallace in Tokyo contributed to this report, which includes material from The Associated Press and Seattle Times archives.