Secretary of State George P. Shultz and Defense Secretary Frank C. Carlucci told the Senate yesterday that the U.S.-Soviet intermediate-range arms treaty will strengthen Western military security as they sought to fend off conservative Republican attacks on the opening day of hearings on the pact.

Presenting a united military front, Adm. William J. Crowe Jr., chairman of the Joint Chiefs of Staff, said the uniformed heads of the Army, Navy, Air Force and Marine Corps support the treaty without demanding offsetting buildups of other arms, as they have done in the past.

Despite assurances from Shultz that the treaty contains strong safeguards against cheating that will be vigorously enforced by the United States, Sen. Jesse Helms (R-N.C.) charged that it is an "engraved invitation to cheat" that the Soviets have already exploited.

Helms' attack came as the Senate's Foreign Relations and Armed Services committees began a month of hearings on the Intermediate-Range Nuclear Forces (INF) Treaty signed last month by President Reagan and Soviet leader Mikhail Gorbachev to strip their arsenals of medium- and shorter-range nuclear-armed missiles.

Most other senators appeared to support the pact or raised reservations that stopped short of indicating outright opposition, reinforcing Senate leaders' predictions that the treaty is likely to be approved without crippling provisos, probably within the next two or three months.

However, several senators suggested conditions on sensitive issues, including a moratorium on buildup of strategic nuclear weapons during START talks on reduction of long-range missile arsenals and language aimed at reducing the Warsaw Pact's advantage in conventional forces in Europe.

Waving a large red-and-white document stamped "Top Secret," Helms, ranking Republican on Foreign Relations and a vocal foe of the treaty, said he obtained classified information last Wednesday raising serious questions about whether the Soviets have already violated its provisions.

When a Helms aide carried the document to Shultz, the secretary refused to open it, protesting it would be improper to do so at a televised hearing. "Can I get this document out of my hands?" he finally asked, and the aide took it back to Helms unopened.

While Helms declined to disclose its contents, Senate sources claimed the document is a new, highly classified estimate of Soviet theater nuclear forces prepared in recent weeks by the U.S. intelligence community.

The sources said it contains information suggesting the Soviets have hidden a "secret force" of SS20 missiles at sites that may not be subject to U.S. inspection under the INF treaty provisions. The information is drawn in part from estimates by the Defense Intelligence Agency, an arm of the Pentagon, that the Soviets produced, deployed and stored more SS20s than suggested by the data exchanged in the final stage of INF treaty negotiations.

Helms claimed that CIA Director William H. Webster confirmed in a letter that information in the document is "substantially accurate." But Sen. Paul S. Sarbanes (D-Md.) quoted Webster as adding that the information by itself does not "constitute a sufficient basis on which to draw conclusions about the overall monitorability of the treaty," and Shultz said he was sure "the problems can be dealt with satisfactorily."

After reading the document during a lunch break, Sen. Alan Cranston (D-Calif.) said it contains 1 1/2 pages of excerpts from three volumes of intelligence data and is filled with "hedges and qualifications," leaving him "far from convinced" that the treaty has been compromised.

Shultz and Carlucci testified that many intelligence analysts believe the Soviet estimates were roughly accurate. "We have no solid grounds for believing that the {Soviet data} is inaccurate at this point," Carlucci said.

To resolve the dispute, Foreign Relations Committee Chairman Claiborne Pell (D-R.I.) said the panel will hear Webster on the issue in a closed hearing Friday.

The treaty, which requires a two-thirds vote of the Senate for ratification, calls for elimination over three years of superpower nuclear missiles with ranges of 315 to 3,125 miles, accounting for 4 percent of current nuclear arsenals.

In his daylong testimony before the Foreign Relations Committee, starting with a reading of a 48- page history of the INF negotiations, Shultz said the treaty was negotiated in a spirit of "tough-mindedness" to bolster U.S. and NATO security while paving the way for broader arms agreements.

"The INF treaty strengthens U.S. and allied security," he said. "It enhances international stability. It may be opening a new chapter in arms control -- the beginning of reductions. It reduces nuclear weapons, rather than setting guidelines for their future growth. It achieves U.S.-Soviet equality by eliminating substantially more Soviet weapons than American ones."

Apparently anticipating conservatives' complaints about Soviet compliance, Shultz said the United States is prepared to react forcefully if the Soviets are caught cheating.

With requirements for the Soviets to destroy more missiles at the start than the United States, Washington has leverage to force compliance because it can "call a halt" to dismantling of its weapons at any time and still have some weapons in place, he said.

Shultz acknowledged that the Soviets have violated treaties but added, "Their violations have been selective and specific. We must be prepared to respond specifically and selectively, with the focus squarely on what is in our own interests."

He said the United States stopped observing the unratified SALT II treaty in response to Soviet violations of the pact and added, "This administration has shown it is ready to take action in case of violations."

In another clash with Shultz, Helms suggested that the secretary had misleadingly suggested that "warheads" were being dismantled by the treaty despite the fact that nuclear explosive devices and their guidance systems can remain intact for possible use on other missiles as INF launchers are destroyed.

Shultz said the terms are sometimes used interchangeably but emphasized that the missile, "whatever you call it . . . is destroyed."

At the end of the hearing, Helms issued a statement contending that Shultz's testimony had been "somewhat confusing" and indicated he will continue pressing on the issue.

During the hearing, Sen. Joseph R. Biden Jr. (D-Del.), chairman of the panel's European affairs subcommittee, suggested adding a proviso to the treaty seeking to prevent the United States or the Soviets from building more long-range missiles to compensate for the loss of INF weapons while START negotiations are under way. He also proposed language that would nail down interpretation of the treaty to bar repetition of the recent fight over testing space-based missile defenses under the 1973 Antiballistic Missile (ABM) Treaty. Shultz was noncommittal on both proposals.

Senate Minority Leader Robert J. Dole (R-Kan.), using the hearings to showcase his campaign for the Republican presidential nomination, cajoled the Foreign Relations Committee into letting him appear as a "guest" to endorse the pact while urging steps to punish Soviet violations, beef up verification capabilities and strengthen non-INF forces in Europe.

In the Armed Services Committee hearing, Crowe was asked by Sen. John W. Warner (R-Va.) whether any member of the Joint Chiefs opposed the treaty. "We are in unanimity," Crowe responded, adding that "the military leaders of NATO are in unanimity."

Crowe's prepared remarks ran 16 pages, twice as long as Carlucci's, and contained a long list of military advantages that were attributed to the INF pact.

Concerns expressed by members of the Armed Services panel included whether the provisions could be verified to catch Soviet cheating; whether Soviets understood that NATO nations could build nuclear missiles of less than a 310-mile range and deploy them in Europe, and whether NATO nations would have the will to build up conventional forces.

Committee Chairman Sam Nunn (D-Ga.) later said he will wait until future hearings have addressed such concerns to decide how to vote on the INF pact.

He said the assurances Carlucci and Crowe gave yesterday about the legitimacy of introducing new nuclear arms in Europe under the treaty did not satisfy him.

Staff writers R. Jeffrey Smith and Eric Pianin contributed to this report.