The Soviet Union is constructing new silos at two missile test sites, indicating that Moscow may be ready for initial flight testing of two new, large intercontinental ballistic missiles (ICBMs) next year, government sources said yesterday.

U.S. intelligence has been tracking development of both missiles -- dubbed the SSX26 and SSX27 by one U.S. source -- since open-air test firing of their engines was observed and publicized several yearsago.

But steady progress on the two missiles, as two other new Soviet mobile ICBMs are being deployed, illustrates Moscow's determination to continue modernizing Soviet strategic forces, intelligence sources said. That modernization is continuing despite an agreement last month in Geneva by President Reagan and Soviet leader Mikhail Gorbachev to pursue a 50 percent reduction in nuclear forces.

One of the untested ICBMs would replace the giant, liquid-fueled SS18, sources said. The other appears to be a smaller, solid-fueled missile about the size of the MX missile being built by the United States.

Disclosure of the test silo construction is contained in the third annual White House report on Soviet arms violations that will be released Monday. That report will charge the Soviets with an "expanding pattern" of violations, according to government sources. A former Defense Department official noted yesterday that the report will be released a week before the unratified SALT II treaty is due to expire on Dec. 31.

Some administration officials want Reagan to announce that the United States will no longer be bound by the terms of SALT II since the Soviets allegedly are violating them.

Other officials want Reagan to continue the policy of not undercutting the agreement while making "proportional" responses to Soviet infractions.

One new issue raised in the pending report, one source said, is Soviet concealment by camouflage of the size of the new ICBM test silos, which prevents American satellites from photographing the silos. Without those photographs, U.S. analysts cannot determine the width and diameter of the silos, data helpful in determining whether the new ICBMs represent potential violations of arms agreements, the source said.

"These may or may not be new missiles that would violate SALT II limits," one former Air Force official said yesterday, "but it is clear the Soviets are not going to help us make that decision."

Another Soviet concealment issue raised in the report concerns the two other new mobile Soviet missiles now in production and the initial stages of deployment.

The United States alleges that the road-mobile, single-warhead SS25 and the rail-mobile, 10-warhead SS24 violate provisions of the SALT II agreement that require the superpowers to reveal both the missile and its launcher during testing.

The report argues that the two new missiles are improperly "concealed" in canisters that are then carried by mobile launchers. One government source said yesterday that U.S. intelligence has never photographed the SS25 missile outside its canister.

Two other new violations listed in the report appear to be technical rather than militarily significant.

The Soviets are alleged to have exceeded the limit they set for themselves in strategic launchers with the deployment of 45 SS25s and 30 new Bear bombers capable of carrying air-launched cruise missiles.

Moscow allegedly failed to cut other weapons to remain within treaty limits.

The Soviets removed more than 50 SS11 missiles from their silos and blew up the silo covers to compensate for the new SS25s, but the silos were not destroyed as required by treaty agreements.

They also put 30 old Bison bombers in a remote airfield and cut the tails off some to prove they were inoperable. But, the report notes, since there are no agreed-upon procedures for dismantling strategic bombers, the Bisons cannot be counted as out of service.

The United States and Soviet Union historically "have had quarrels of timing and character of procedures for destroying weapons," the former Air Force official said. "These were always worked out at the SCC the Geneva-based Standing Consultative Commission established by the SALT I treaty ," he added.

The report also notes that the Soviets have taken action on some older alleged violations.The production rate of Backfire bombers, which last year was called a violation of an agreement to build only 30 a year, is called "ambiguous" in the new report.

There also is "some evidence," one source said, that SS16 mobile missile equipment was moved from a base where last year it was reported as being deployed in violation of the SALT II pact.

In another area, sources said, the report notes that the Soviets appear to have halted use of "yellow rain" toxins in Afghanistan, also the subject of a U.S. violations complaint last year.