MOSCOW, AUG. 11 -- A senior arms control official today confirmed that the Soviet Union is deploying a new mobile intercontinental missile system and said Moscow's intention was "to increase the stability" of its nuclear forces.

Victor Karpov, the head of the arms control and disarmament directorate of the Soviet Foreign Ministry, said the deployment of the new missile, known to western analysts as the SS24, was a "modernization" of existing Soviet strategic forces. He said the move does not violate the provisions of the unratified SALT II treaty on nuclear arms between the Soviet Union and the United States.

The mobile missile, he said, was intended "to raise our survivability, the survivability of our forces" in the event of a first nuclear strike by the United States. As a result, he maintained, "this gives us additional stability" and should not increase tensions between the superpowers.

Karpov did not say how many of the new missiles had been deployed or whether the action had been completed, stating only that "we are carrying out" the replacement of older missiles with the new system.

The official's statement at a news conference came in response to reports from Washington last weekend on the missile's deployment. It appeared intended to deflect western criticism that the move represented an escalation of the nuclear arms race at a time when Moscow is seeking an agreement to eliminate Soviet and U.S. intermediate range missiles.

Soviet officials informed the United States several years ago that the SS24 would be deployed as the one new missile system permitted by SALT II, and the Pentagon had anticipated that a rail-mobile SS24 would be deployed during 1987. However, western diplomats here said Karpov's statement was the most direct public confirmation that the missile was being deployed.

According to the Pentagon, the SS24 can carry up to 10 independently targeted nuclear warheads, and its range of 6,200 miles would enable it to reach major U.S. military targets through polar routes.

U.S. officials said last week that the SS24 would be transported by railroad cars, making it more difficult to locate and destroy in the event of a nuclear conflict. Karpov did not specify how the missiles would be moved, confirming only that they would be mobile.

The United States does not have a mobile strategic missile system. The Reagan administration recently began studying the deployment of new MX missiles on railroad cars and is still developing a single-warhead missile that could be carried by truck.

Karpov denied western suggestions that the SS24 deployment would violate SALT II, saying that Moscow would stay within the treaty's limits for the total number of land-based and sea-based missiles and heavy bombers. The United States ceased observing SALT II limits on land- and sea-based missiles last year by deploying nuclear-tipped, air launched cruise missiles.

Sen. Jesse Helms (R-N.C.) charged last week that the SS24 would violate SALT II limits because not all of the SS17 missiles it was replacing had been deactivated. U.S. officials responded that Moscow appears to remain within the treaty limits on overall numbers, but may not have covered all of the silos used for the SS17.

American officials also maintain that the Soviet Union has violated SALT II by deploying the SS25, another long-range mobile weapon borne by truck. Soviet officials argue that the SS25 is not a new missile system, but merely an upgrading of a previous rocket.

While describing the new missile deployment today, Karpov reiterated the Soviet position that Washington and Bonn are blocking the way to an agreement to eliminate the intermediate forces by refusing to agree to the removal of 72 aging Pershing IA missiles based in West Germany.

Karpov also said that a compromise suggested by some western officials involving the gradual retirement of the Pershings would not be acceptable to Moscow, which is concerned about the U.S.-controlled warheads. "We do not put the emphasis on the missiles, which belong to the Federal Republic of Germany," he said. "We speak about the warheads. The warheads must be eliminated."

The Soviet official said that "we have offered everything on our part possible" to reach agreement in the negotiations in Geneva. "Now it depends on the United States," he added.

"If the negotiations are broken then they are to blame."