The Soviet Union is believed to have started deploying the world's first strategic nuclear missile able to be launched from railroad cars, U.S. officials said yesterday.

The new rail-mobile missile, known to western analysts as the SSX24, can be moved on vast portions of the Soviet rail system to elude U.S. monitoring and attack, officials said, potentially complicating future efforts to verify Soviet compliance with arms-control treaty limitations.

Deployment of the missile on rail cars and in silos has long been expected by the U.S. intelligence community. Some Reagan administration officials have attacked it as a potentially destabilizing development in the arms race, although independent experts disagreed.

U.S. officials said several SSX24 missiles have been moved on rail cars from the factory where they were produced, but they disagreed about whether the missiles had yet become fully operational.

Its development has been praised by some U.S. arms-control experts as an important shift from vulnerable silo-based intercontinental ballistic missiles. These experts argue that both sides benefit by having nuclear forces capable of being hidden from the other side, making them less vulnerable to a devastating first strike.

U.S. confirmation of the ICBM deployment came yesterday afternoon in response to a statement on the Senate floor by Sen. Jesse Helms (R-N.C.), alleging that the Soviets had violated a key provision of the unratified SALT II treaty by deploying the missiles.

U.S. officials said the White House distributed a directive suggesting that reporters be told the Helms allegation was "essentially correct" and that the deployment should be regarded as a "sobering" development.

A knowledgeable U.S. official said that while some U.S. intelligence experts dispute the allegation, "the preponderance of the intelligence community believes that the Soviet Union has begun deploying the rail-mobile SS24."

The SSX24, in development more than a decade, can carry up to 10 independently targeted nuclear warheads, the Pentagon says. Its range of 6,200 miles enables it to reach most major U.S. military targets via polar trajectories.

Helms, who sought to discredit an expected move by Senate Democrats to require through legislation that the United States adhere to the SALT II treaty so long as the Soviets remain in compliance, said he obtained agreement from "key intelligence, defense and national security authorities" that "some SSX24 . . . rail-mobile launchers should now be accountable" under the SALT II limit on land-based missiles with multiple warheads.

The United States ceased observing the SALT II treaty late last year when it deployed nuclear-tipped, air-launched cruise missiles in excess of a limit on land- and sea-based missiles with multiple warheads. It did so in response to alleged Soviet violations of other SALT II treaty provisions.

An exact assessment of the state of the new Soviet missiles was said to be complicated by the fact that U.S. intelligence experts are divided about their ability to detect which Soviet rail cars can be equipped to carry the missiles, despite months of study driven by the Pentagon's desire to be able to locate and hit the rail cars in event of a nuclear conflict.

One U.S. official said the Soviets indicated that deployment of the new missile was imminent a month or so ago by officially notifying the administration under a provision of the SALT II treaty that it had completed a test program. But another official said he was unaware of this notification.

The Soviet Union had previously deployed another mobile strategic nuclear missile known as the SS25, designed to be launched from trucks that roam the Soviet countryside. The United States, in contrast, has just begun studying deployment of its 10-warhead MX missiles on railroad cars, and is still developing a single-warhead, road-mobile counterpart to the Soviet SS25.

The Reagan administration's special commission on strategic nuclear forces, chaired by former national security adviser Brent Scowcroft, urged that both superpowers shift to mobile, relatively untargetable, land-based missiles so as to lessen the attraction of striking first when conflict seems imminent.

But the Reagan administration has argued instead that Soviet compliance with limitations on mobile missiles would be so difficult to verify that none should be permitted in the treaty constraining strategic nuclear arms now under negotiation in Geneva.

The high cost and difficulty of verifying mobile-missile deployments was also a major impetus for the administration's successful effort to get Soviet agreement that all mobile medium- and short-range missiles be banned in any treaty on U.S. and Soviet forces in Europe and Asia. The two countries have reached agreement on all but a few issues in a proposed treaty eliminating the medium- and short-range missiles.

Several U.S. officials challenged Helms' allegation of a serious new Soviet SALT II treaty violation, and asserted that the Soviets have dismantled a sufficient number of SS17 missiles to make up for recent new deployments of the SS24 and a new missile-carrying Typhoon submarine.

The officials explained that Helms' assertions were technically correct, however, because the Soviets had not yet blown up the old SS17 silos as required by the SALT II treaty, even though the missiles had been withdrawn.

Sen. Dale Bumpers (D-Ark.), a proponent of legislation demanding U.S. and Soviet compliance with SALT II, said that "if true, the allegations indicate we are reaping the predictable response" to the administration's decision to abandon SALT II. The issue is expected to come before Congress in late September.