Defense Secretary Caspar W. Weinberger said yesterday that the Soviet Union has deployed a new, mobile intercontinental missile, which he called an "unquestionable violation" of the SALT II arms-control treaty.

Soviet deployment of the SS25 missile has long been expected this year by U.S. intelligence officials. Weinberger's disclosure, in a speech sharply critical of Moscow's arms-control record, comes amid intense superpower jockeying for world public opinion before President Reagan's summit with Soviet leader Mikhail Gorbachev next month.

"I can officially confirm that one of the new ICBMs, the mobile SS25, is now being deployed and is an unquestionable violation of Soviet assurances given to us under the SALT II accord," Weinberger said in remarks to the Ethics and Public Policy Center here.

The SALT II treaty limits each superpower to one new, long-range missile. The United States is developing the multiwarhead MX missile. The Soviets have claimed the large, mobile, 10-warhead SSX24 as their new missile, which U.S. intelligence officials predict will be deployed in silos next year and on rail-mobile launchers in 1987.

Moscow denies that the single-warhead SS25 is a new weapon, calling it an an updated version of the older SS13 missile based in underground silos. But U.S. officials insist that the SS25 is completely different from the SS13.

"The SS25 violates the SALT II agreement that permits development of only one new type of ICBM," Weinberger said. "Their first new type developed, the SSX24, is now being tested."

In his condemnation of the SS25 deployment, Weinberger foreshadowed a key Defense Department review of alleged SALT II treaty violations to be delivered to Reagan by Nov. 15.

Reagan decided in June to continue observing the SALT II accord, which expires Dec. 31. But the president directed the Pentagon to catalogue Soviet treaty violations and to recommend "proportional responses."

Weinberger called the SS25 an "extremely versatile weapon," a 20-yard long missile with an approximate range of 6,500 miles. It will be stored in launcher garages with sliding roofs and moved by road from site to site to complicate U.S. targeting, he said.

U.S. officials said Soviet placement of the SS25s was recently confirmed in intelligence reports, a few weeks after Soviet negotiators at the Geneva arms talks announced their deployment plans.

Some U.S. intelligence officials believe the SS25, if swapped with SS13s, is less threatening because mobile missiles are less accurate than those based in underground silos. Pentagon officials said yesterday that while the SS25s are being deployed at least temporarily in older intermediate-range missile silos, there is "no evidence" that they are replacing the SS13 missiles.

Other officials call the SS25 a dangerous escalation of the arms race, claiming that its mobility makes it more difficult to track for arms control purposes.

Weinberger did not specify either the location or number of deployed SS25s. U.S. officials have previously reported Soviet construction of concrete shelters for 24 of the missiles at the Yashkarola missile base and another 24 at the Yurya base.

Weinberger, in yesterday's speech, offered a generally dark view of Soviet arms-control intentions, citing alleged violations by Moscow of the SALT I and Anti-Ballistic Missile (ABM) treaties.

"Recent history shows that arms control has hardly been a raving success," he said.

Given the failure of previous arms-control agreements to reduce nuclear weapons, he said, "it is difficult to argue that the only moral course of action open to the United States is more of the same.

"There is nothing moral about a situation in which the strength of the democratic nations is slowly eroded," he added.