The Soviet Union is modifying some missile silos in a way that could permit it to deploy its new mobile, multiwarhead SSX24 intercontinental ballistic missile next year without violating the SALT II agreement, according to sources in the administration and on Capitol Hill.

The Soviets are "sending a signal that they are ready for deployment but don't have to go outside SALT," one defense official said, adding that they also are showing "the capability to exceed the SALT numbers and put mobile missiles out in large numbers."

Neither country has ratified the treaty, whose terms expire at the end of the year, but both have said they will not undercut its provisions.

President Reagan, however, has charged that the Soviets have violated some provisions of SALT II. In a report to Congress in January, he said the Soviets had begun production of two new missiles although the agreement permits them only one, and he accused Moscow of violating the provision that prohibits encoding missile-test information.

Now, sources said, Soviet preparations may have begun to replace old four-warhead SS17s with SSX24s, which could have as many as 10 warheads.

In addition, analysts expect the SSX24 to be carried on a special railroad launcher, from which it has been test-fired.

If the Soviets substitute SSX24s for SS17s, they will avoid violating the multiwarhead missile limit.

Both the United States and the Soviet Union are close to exceeding SALT II limits on multiwarhead intercontinental ballistic missiles, ICBMs.

The Soviets, with 818 land-based, multiwarhead ICBMs in silos, are two missiles under the limit for such weapons and are preparing to deploy SSX24s.

The U.S. total of land- and sea-based multiwarhead missiles is 1,190, which is 10 under the treaty limit for such missiles. In August, when the Trident submarine USS Alaska is scheduled to go on sea trials with 24 multiwarhead Trident ICBMs, the United States will exceed the 1,200 limit by 14.

Under past treaty compliance practices, the United States will have 60 days after the Alaska goes to sea to begin reducing its multiwarhead missile force to the limit.

The U.S. Navy has set aside funds to dismantle a Poseidon submarine, which carries 16 missiles, to stay under the limit. In the past, both countries have destroyed early-model ballistic missile subs under the agreement's provisions.

By June 1, the president must send Congress a report on how he plans to adhere to the SALT II limits. Reports from administration sources conflict as to whether that document will disclose how the United States will act or will simply describe options available to the president and reserve the decision for the fall.

Defense Secretary Caspar W. Weinberger said yesterday that the administration is "considering right now different options" about what to do with the SALT II limits.

In an appearance on NBC-TV's "Meet the Press," Weinberger refused to give an opinion of how Reagan should decide the issue. But he reiterated his view that the treaty "has not operated to reduce armaments." He said that "rather than talk about an old agreement," he wanted an agreement that sharply reduces nuclear systems.

In Geneva last month, Soviet officials told U.S. representatives about another exchange that they said would keep them within the SALT II limits.

They said they would remove SS11 single-warhead ICBMs from silos and introduce newer single-warhead SSX25s, which may be carried on trucks or placed in silos. The Soviets claim that the SSX25 is a modified SS13 and thus is permitted under SALT II. Some American officials have contended that the SSX25 is really a second new missile and thus prohibited by the treaty.

"The Soviets are keeping their options open on staying under SALT II limits to see what we do," one congressional defense specialist said.

Meanwhile, Sens. Jesse Helms (R-N.C.) and Steve Symms (R-Idaho) plan to introduce an amendment to next year's defense authorization bill that would require the president to certify that the Soviets are in compliance with SALT II before any U.S. missiles are dismantled in response to the Alaska's sea trials.

Since Reagan has reported several violations of SALT II, the aim of the legislation is to prevent dismantling of U.S. missiles.

Administration sources have said officials are discussing a plan to put the submarine in drydock, rather than dismantle it. Such a "gray area" step would imitate some past Soviet actions and would not openly violate the agreement, officials say.

Sen. Dale Bumpers (D-Ark.) and 18 colleagues are cosponsoring a nonbinding, sense-of-the-Senate amendment to the defense bill that calls on the president to continue U.S. compliance with the SALT II limits.