For half a decade, Russia has conducted what it says are nonnuclear tests under the ground of an Arctic island, as the United States says it does beneath the Nevada desert.

But the tests have caused bitter divisions among intelligence officials and nuclear analysts in Washington. Some have concluded that Russia is lying and is instead detonating small nuclear blasts; other experts say that charge is reckless and probably wrong.

"This question," one intelligence analyst said, "is tearing the community into pieces."

Beyond the dispute is the question of what, if anything, to do if Russia is lying. Led by Republicans, the Senate rejected the global ban on nuclear explosions and it is unclear whether the United States would now accuse Russia of violating it.

Paradoxically, the rejected test- ban treaty had provisions for inspections by which the United States could have sought to examine the Russian test site.

Still, Russia's truthfulness is relevant since underground tests serve to perfect new kinds of nuclear warheads.

President Clinton was briefed on the dispute shortly before he left office and the Bush administration is reviewing it, federal scientists and officials said. The White House offered no view. "We're not going to comment on intelligence matters," said Mary Ellen Countryman, a spokeswoman for the National Security Council.

Russia strongly denies any deception and says the tests comply with permitted practice. And some federal experts called the charge most likely false, saying the evidence is weak and the analyses flawed.

The intelligence analysts behind the charge have a history of suspecting the worst of Russia, and in one case of embarrassing the United States by accusing Moscow of conducting a nuclear blast that turned out to be an earth tremor. Such analysts have criticized the test ban treaty as hard to monitor. Moreover, the nuclear scientists who are taking part in the analyses often oppose bans on testing weapons they have designed, and some have argued for renewed American testing.

But both sides are said to agree that Moscow is doing more at the Arctic island, Novaya Zemlya, than it has acknowledged. "It's certain," a federal official said, "that the announced activity doesn't tell the whole story."

A positive outcome of the current dispute, said a senior federal science adviser, could be more intrusive means of verification at the nuclear test sites of both sides, which might cut through the fog of suspicion. "These are examples," he said, "of why we need more transparency."

The silence at most of the world's nuclear test sites comes after a half century of explosions in which new and old designs were checked to see how well they worked. The ban on such tests seeks to curb arms developments and races.

To diplomatic acclaim, President Clinton signed the test ban in 1996 after championing its adoption. It allows small tests in which nuclear materials are thrown together as long as the experiments have an energy output equal to zero. In other words, "zero yield" experiments are to produce absolutely no burst of nuclear energy, however tiny, and are widely agreed to have no use in designing new warheads.

The dispute centers on an inherently tricky area of test-ban verification in which nuclear blasts have yields too small to produce the kind of powerful shock waves that distant nations can track easily as faint rumbles in bedrock. Because of that, the debate tends to turn on sketchy evidence, worst-case scenarios and skeptical retorts.

Russian officials, in denying any violations of the ban, said military scientists on Novaya Zemlya are doing nothing more than simple experiments far too weak to represent an atomic blast.

In an interview, the head of the development and testing of nuclear weapons at Moscow's Atomic Energy Ministry, Nikolai P. Voloshin, said "We are not violating the treaty, absolutely."

Russia says it is doing so-called subcritical tests that are allowed under the Comprehensive Test Ban Treaty, which 160 nations have signed. While so far unratified, the ban is mostly observed in practice; Moscow has pledged to abide by its rules.

Subcritical explosions use nuclear fuel like plutonium in small discharges that stop short of producing a self-sustaining chain reaction that releases any nuclear energy.

But some federal intelligence analysts charge that Russia is engaging in a type of outlawed test known as hydronuclear. In those tests, metallic bomb parts are thrown together explosively, liquefying (thus the hydro) while releasing small amounts of nuclear energy. The tests stop short of a large blast, releasing perhaps a millionth of the energy of the Hiroshima bomb.

Experts agree that hydronuclear tests can have some use in the design of new nuclear arms, although the extent is debated.

The intelligence team that says Russia is lying includes Lawrence Turnbull, a Central Intelligence Agency analyst, and Charles Craft, a Sandia National Laboratory analyst, officials said.

Mr. Craft leads a panel of the Joint Atomic Energy Intelligence Committee, a group that represents the nuclear views of many federal agencies.

The two, officials added, form the core of a group within the intelligence community that believes that it has evidence that Moscow is going over the hydronuclear line in an effort to develop new kinds of nuclear arms.

Part of the team's evidence, a federal official said, centers on highly sensitive intelligence sources that are seen as giving Washington a clearer view into Moscow's activities on Novaya Zemlya. Neither Mr. Turnbull nor Mr. Craft responded to requests for comment.

Officials said the State Department is skeptical of the accusation and has written formal rebuttals.

The differing sides in the dispute are trying to influence the formal process by which the federal government periodically makes judgments about secret foreign activities. This National Intelligence Estimate seeks to describe the likely state of development in the Russian nuclear program.

Fueling mistrust, officials said, is the sheer bustle on the hilly island, a seemingly barren place about 500 miles long and 500 miles east of Murmansk, inside the Artic Circle. They said American surveillance has observed a flurry of experimental work as well as Russian planes and ships ferrying supplies and nuclear crews back and forth.

"There's lots of interest, activity and money involved," said a top federal science adviser. "So you can understand why people are suspicious."

Mr. Turnbull and his allies have a history of faulty analyses. In August 1997 they told the White House that the Russians might have conducted an underground test at Novaya Zemlya. But after seismic experts challenged that assessment, the C.I.A. retracted that finding and said the tremor was actually a nearby undersea earthquake.

"They've got an ax to grind and are still trying to save face from that," said one federal science adviser.

Defenders of Mr. Turnbull note that Russia has often cheated on arms-control treaties, and that top Russian experts are arguing for new nuclear arms. In Moscow, Viktor N. Mikhailov, a former minister of atomic energy who still wields much power, has been quoted as advocating "a new generation of super precise nuclear weapons."

But Frank von Hippel, a physicist at Princeton University who advised the Clinton administration on the nuclear test ban, said he had heard rumors of the intelligence dispute and considered the violation charge irresponsible. "As far as I can tell," he said, Washington has no evidence "that would prove that the Russian activities are any different than those that the U.S. conducts at the Nevada test site."

If the intelligence team's finding becomes the American view, it might stir a political storm. Even though the Senate in 1999 rejected the test ban by a vote of 51 to 48, the United States is currently conducting no nuclear tests, and weapons experts said the perception of a Russian violation could erode or end support for testing restraint.

Many arms-control experts see small nuclear tests such as those allegedly being done by Russia as too small to be militarily significant, and argue that branding Russia as a test- ban violator on the basis of slim evidence poses more risks than benefits.

But some intelligence analysts argue that Moscow over the decades has learned to tease so much information from small tests that the secretive work could produce new classes of nuclear arms.

Federal experts said the dispute does not appear to be politically motivated or timed to the change of administrations.

Novaya Zemlya is covered with snow and ice most of the year. Starting in 1955, Russia conducted more than 100 nuclear blasts there, the last in October 1990. Since 1995, Russia has used the remote wilderness for what it says are permissible underground experiments to maintain the reliability of its nuclear arms.

From last August through October, Russia announced a series of small tests there.

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