In a new assessment of its capabilities, the Central Intelligence Agency has concluded that it cannot monitor low-level nuclear tests by Russia precisely enough to ensure compliance with the Comprehensive Test Ban Treaty, which the Senate will begin debating this week, senior officials said yesterday.

Twice last month the Russians carried out what might have been nuclear explosions at its Novaya Zemlya testing site in the Arctic. But the CIA found that data from seismic sensors and other monitoring equipment were insufficient to allow analysts to reach a firm conclusion about the nature of the events, officials said.

The Russian government has assured the Clinton administration that the tests involved only conventional explosives and that it has not broken promises to abide by the unratified treaty, which prohibits nuclear tests.

Senior congressional staffers were briefed on the new CIA assessment before Senate Majority Leader Trent Lott (R-Miss.) last Thursday abruptly scheduled a vote on the test ban treaty after having refused to bring it to the floor since President Clinton sent it to the Senate for ratification two years ago.

Lott vowed to defeat the treaty because it endangers U.S. security. Clinton has promised an all-out fight for ratification of what he calls a landmark arms control pact.

Republicans and Democrats predicted yesterday that the CIA's ability to monitor low-level tests will be a major issue in the debate leading up to a vote that could take place as early as Oct. 12. Senior intelligence officials, including possibly Director of Central Intelligence George J. Tenet, will begin briefing senators on the monitoring issue Monday, sources said.

Ratification of a treaty requires a two-thirds vote in the Senate, and by all accounts the Democrats, who hold 45 seats, are far short of the required 67 votes. The treaty has been signed by 154 nations, including the United States, but it has been ratified by only 47 countries, most recently Bulgaria on Wednesday. More significant, the treaty has been ratified by only 23 of the 44 nuclear-capable countries that must confirm it for the treaty to take effect.

Although the U.S. intelligence community has a long-standing concern about the difficulty of gathering data on low-level nuclear tests, the recent Russian tests -- and others like them earlier this year -- prompted the CIA reevaluation. As a result, the agency formed a new assessment that these events fall into a gray area where it cannot reliably distinguish between a conventional explosion and a low-level nuclear test or even natural seismic activity, officials said. U.S. officials said that assessment is not a dramatic departure from earlier CIA positions but rather a refined judgment about its ability to deal with a subject that is inherently uncertain.

"Without the treaty, the problem of assessing these kinds of events undoubtedly exists, but the question you have to ask is whether the treaty would leave us better off or worse, and inarguably we would be better off," national security adviser Samuel R. "Sandy" Berger said in an interview yesterday. Under the treaty, an international monitoring system would be put in place with capabilities exceeding those that the United States and it allies can field today, and signatories would have the right to request on-site inspections of testing facilities, ensuring that compliance can be verified, he said.

While the administration argues that the treaty would provide new tools to detect testing that would help remedy the weaknesses in U.S. capabilities, Republican leaders contend that the treaty is worthless unless the United States can ensure compliance on its own, because Russia, China and other nations have a history of denial and deception on nuclear testing.

During a speech to the Senate on Friday declaring his opposition to the treaty, Armed Services Committee Chairman John W. Warner (R-Va.) said that the recent history of Russian testing activity had to be taken into account. "There is a body of fact developed over the past 18 months that it will be imperative for every senator to examine before deciding how to vote," Warner said in an interview. That information would be made available to the Senate during briefings and hearings this week, Warner said.

According to a military intelligence assessment that has circulated widely at the Pentagon and in the intelligence community, over the past 18 months Russia has conducted tests in the granite caverns of Novaya Zemlya to develop a low-yield tactical nuclear weapon that is the linchpin of a new military doctrine to counter U.S. superiority in precision guided munitions.

In monitoring Novaya Zemlya, U.S. surveillance satellites have repeatedly observed the kind of activity that usually precedes and then follows a low-level nuclear test; in between, seismic data that are gathered have been insufficient to allow a clear assessment of what transpired, officials said.

"We do not have any data that indicates a nuclear explosion during those events," said a senior administration official.

The administration's position is that Russian President Boris Yeltsin has stood by his 1997 promise to conduct only "subcritical" tests, in which conventional explosives are detonated in the presence of nuclear materials as a way of testing existing nuclear weapons without creating a nuclear chain reaction. The United States, which stopped nuclear testing in 1992, also has used subcritical tests to evaluate weapons.

Although some officials at the CIA and other intelligence agencies believe that Russia has repeatedly conducted nuclear tests in violation of Yeltsin's promise, the CIA does not claim to have conclusive data one way or the other. Indeed, it is uncertainty about what is happening rather than an accusation of Russian misbehavior that is the key point of the CIA assessment, officials said.

"Tests at these kinds of levels are difficult to characterize in an exacting manner, and that is a major challenge to the intelligence community," a senior U.S. official said.

The administration is prepared to argue that the difficulty of monitoring low-level tests is a major factor in favor of the treaty and its new global monitoring system, but administration officials are concerned that their message will take longer to get across than the stark suspicions of Russian motives that lie behind many Republican arguments.

"It is unfortunate that after two years of inaction we now get a 12-day rush to judgment," Berger said.

"We don't think this is a good treaty," Lott said Friday. "We think it would put us in a weakened position internationally, but since there have been all these calls and demands for a vote, we have offered to vote."