U.S. intelligence agencies say, based on preliminary indications, that North Korea did not produce its first nuclear blast yesterday.
U.S. officials, speaking on the condition of anonymity, said that seismic readings show that the conventional high explosives used to create a chain reaction in a plutonium-based device went off, but that the blast's readings were shy of a typical nuclear detonation.
"We're still evaluating the data, and as more data comes in, we hope to develop a clearer picture," said one official familiar with intelligence reports.
"There was a seismic event that registered about 4 on the Richter scale, but it still isn't clear if it was a nuclear test. You can get that kind of seismic reading from high explosives."
The underground explosion, which Pyongyang dubbed a historic nuclear test, is thought to have been the equivalent of several hundred tons of TNT, far short of the several thousand tons of TNT, or kilotons, that are signs of a nuclear blast, the official said.
The official said that so far, "it appears there was more fizz than pop."
A successful nuclear detonation requires a properly timed and triggered conventional blast that splits atoms, setting off the nuclear chain reaction that produces the massive explosions associated with atomic bombs.
White House spokesman Tony Snow said assessing the validity of North Korea's claim of a successful nuclear test could take several days.
"We need to find out precisely what it is that took place yesterday, and that is something that's going to take awhile for the scientists and others to work through," Mr. Snow said.
"Nobody could give me with any precision how long it will take until they can say with certainty what happened."
Nuclear bombs make big waves, with clear signatures that make them fairly easy to detect, analyze and confirm that they were caused by splitting atoms. But smaller blasts -- as North Korea's appears to have been -- are trickier to break down, scientists told the Associated Press.
"It takes days, dozens of lab hours, to evaluate results. Now we can have only a rough estimate," said Russian nuclear physicist Vladimir Orlov of the Moscow-based Center for Policy Studies in Russia, a nonproliferation think tank.
Elements of the blast were detected by U.S. and allied sensors as it was set off in an underground tunnel in the north-central part of North Korea. U.S. intelligence agencies have been monitoring several tunnels thought to be nuclear test facilities and have not ruled out Pyongyang's conducting another test.
U.S. officials said the test was timed to coincide with several anniversaries in North Korea, including the end of mourning for the death of North Korean leader Kim Jong-il's father, Kim Il-sung. The test was thought to have been linked to the commemoration.
North Korea's military thinks that joining the world's seven other acknowledged nuclear powers is key to preserving the power of the communist regime.
There were wide variations in seismic data of the North Korean blast. The French atomic agency estimated about 1 kiloton, and South Korea's geological institute said half of that. But Russia's defense minister expressed "no doubt" that North Korea detonated a nuclear test and said the force of the underground blast was equivalent to 5,000 to 15,000 tons of TNT.
"People have different ways of cross-cutting the data and interpreting them," said Lassina Zerbo, director of the International Data Center at the nuclear-test-ban preparatory commission, which is based in Vienna, Austria.
The Bush administration is pushing for the United Nations to adopt economic sanctions against North Korea that would include a blockade of all goods moving into and out of the country.
Key to the imposition of the tough sanctions will be support from China and Russia, two states that in the past opposed sanctions.
The most immediate impact of the underground test is that U.S. officials fear Japan will take steps to develop nuclear weapons as a deterrent.
Official North Korean press for the past several years has been asserting that the United States is planning a pre-emptive nuclear attack on North Korea over its secret uranium-enrichment program.
Intelligence reports from several years ago indicated that North Korea was engaged in a covert program to develop a uranium-based nuclear program with the help of Pakistani nuclear scientist Abdul Qadeer Khan. The Khan network supplied centrifuges and nuclear-weapons design techniques to Libya, Iran and North Korea.
What U.S. officials have been unable to confirm is whether North Korea received small warhead design information from the Khan network.
Chinese-language documents on how to build a nuclear warhead for missiles were found in Libya and were supplied by Khan network associates. U.S. intelligence officials think Iran and North Korea received similar warhead design documents.
North Korea in July conducted flight tests of seven missiles including a long-range Taepodong-2.
U.S. officials think the plutonium for the pit of the North Korean device was produced by the reactor at Yongbyon, the regime's declared nuclear facility.
U.S. intelligence agencies estimate that North Korea has some 88 pounds of plutonium and that about 13 pounds were used in the recent test.
The remaining plutonium is enough for North Korea to make about six bombs.
Joseph Curl contributed to this article, which is based in part on wire service reports.