The Soviet Union has told the United States in Geneva that it will replace older SS11 intercontinental ballistic missiles now in silos with new SS25 ICBMs to avoid undercutting what Moscow considers to be its limits under the unratified SALT II strategic arms limitation agreement, according to informed sources.

The Soviets said they would initially deploy 18 of the new, mobile, single-warhead SS25s and remove 20 SS11s from silos, sources said.

Soviet officials discussed the missile exchanges two weeks ago before the U.S.-Soviet standing consultative commission (SCC), whose normally secret sessions deal with questions about adherence to terms of arms control agreements. Word of the Soviet move came from persons inside and outside of government who are critical of what they say is the Reagan administration's preparation for the United States to break out of the treaty limits.

President Reagan is required by law to report to Congress by June 1 on the consequences of continuing U.S. adherence to the SALT II limits. An interagency committee is developing options for him.

The administration is also pressured by the fact that the United States could exceed the SALT II limits for multiwarhead missiles in October. At that time, the U.S. Navy is scheduled to deploy a new Trident submarine whose 24 missiles would take the United States 14 missiles above the limit if no compensatory step is taken, such as retiring a 16-missile Poseidon submarine or deactivating 14 Minuteman III land-based ICBMs.

On Tuesday, Assistant Secretary of Defense Richard N. Perle, a key arms control policy-maker in the Pentagon, told a Senate Arms Services subcommittee that in his "personal view," the United States should not continue to respect the treaty limits after the agreement expires at the end of this year. Although the SALT II treaty was never ratified, both superpowers agreed to respect its limits.

Perle argued that the United States had more to lose than the Soviets by adhering to the treaty over the next few years. Because of the Trident submarines due to be deployed over the next few years and the 10-warhead MX ICBM due in December 1986, the United States would have to retire a "significantly larger number" of missiles than the Soviets, Perle said.

Others disagreed. Yesterday, Randall Forsberg of the Institute for Defense and Disarmament Studies said a study of Soviet weapons programs showed that Moscow has the most to lose from the SALT II limits because it was close to the limits for land- and sub-based missiles, and had many new missiles coming on line that could only be deployed by retiring others, if the treaty limits continue to be respected.

In discussing the Soviet SS25 presentation in Geneva, one expert said yesterday, "They are sending us a mixed message. They say they are interested in continuing interim restraints on missiles, but they want us to know they can expand their offensive forces rapidly" if the SALT II limits are dropped.

The SS25 came up for discussion, one source said, because Washington said the SS25 missile violated the SALT II treaty provision limiting each country to one new ICBM. The Soviets responded that the SS25, which also can be carried on a truck-like mobile launcher, is not a new missile but a modification of its earlier SS13 ICBM.