The Soviet Union apparently is planning to flight test a new, solid-fuel mobile missile, which Pentagon sources say raises the possibility that Moscow will violate the limitations on new ICBMs in the 1979 SALT II treaty.

Preparation for the long-expected launch, including emplacement of mobile launching equipment, has been observed the past few months at the Soviet missile test site near Plesetsk, 600 miles northeast of Moscow, sources said.

Intelligence sources are unable to determine what the range of the new missile would be, which is crucial to the question of whether it would violate SALT II. The Soviets have been working for more than five years on two new solid-fueled mobile missiles, according to a new book on Soviet strategic forces published this month by the Brookings Institution.

One is a replacement for the intermediate-range SS20 mobile missile, 333 of which have been deployed in various parts of the Soviet Union over the past five years. This missile would not violate SALT II.

The other is a follow-on to the intercontinental-range SS16 mobile missile, which has never been made operational and was banned under terms of the SALT II. Its deployment would violate the SALT II provisions, according to Pentagon sources.

The 1979 agreement has not been ratified by the United States, but both Moscow and Washington have said they would abide by its provisions, one of which limited each side to one new intercontinental-range missile.

On Oct. 26, the Soviets test-fired a large solid-fuel missile, and Soviet Ambassador Anatoliy F. Dobrynin informed the State Department that it was to be his country's one new ICBM that was allowed by SALT II. Thus, testing of the second new missile, the proposed intercontinental follow-on to the SS16, would be a violation of SALT II, according to Pentagon sources.

U.S. sources said the missile tested in October was the same size as the current SS19, a long-range intercontinental missile that is larger than any U.S. missile, including the proposed MX. The new Soviet rocket was powered by solid fuel rather than liquid fuel as are all other silo-based Soviet ICBMs. The new missile failed after the first of three stages ignited.

By moving to solid fuels, the Soviets will be able to have more missiles on alert and be quicker to respond to a launch command. All U.S. Minuteman missiles are solid fueled, and the 51 remaining old Titan II missiles, which are being retired, are liquid fueled.

A non-governmental specialist in nuclear weaponry said yesterday that Soviet testing of a new mobile missile could be "an ominous sign" that marks the beginning of a "new round of arms escalation."

If the Soviets described the new mobile missile as an intermediate-range missile, it would not be governed by the SALT II limitation provisions and would not be a violation of the treaty.

It also could be described as a modernization of the SS16, which Pentagon officials argue would be a violation of SALT II. Modifications of existing missiles are permitted by the treaty as long as they do not vary by more than 5 percent in size and throw-weight from the existing missile.

Pentagon sources said, however, that the Soviets cannot modify the SS16 since it was tested but not deployed. Thus, one Defense Department official recently argued, a new Soviet mobile ICBM would be an entirely new missile under terms of the treaty.

"This may be the stick behind Soviet leader Yuri V. Andropov's recent proposal for a reduction in European missiles," one source said. Both Andropov and Soviet Defense Minister Dmitri Ustinov have said in recent statements that the Soviet Union would respond to new U.S. missiles with new weapons of their own.

The Brookings Institution analysis of Soviet missiles concludes that "the one new development that is likely for improving Soviet ICBM survivability is the deployment of mobile ICBMs, as a supplement to a larger silo-based force . . . . "

During a recent conversation, a Soviet diplomat in Washington said his government's testing of the new mobile missile would be no more of a violation of the SALT II agreement than the proposed deployment of new U.S. MX missiles in hardened silos in the Dense Pack basing scheme. The Soviets argue that building new silos is a violation of SALT II, and he added that, faced with such an action, his country would not feel restrained from testing new missiles.

The proposed MX missile, with its 10 warheads, would threaten the Soviet silo-based missile force, according to Pentagon officials. Proponents of the MX have encouraged its development and deployment as a means of forcing the Soviets to dismantle their large ICBMs, such as the SS18 and SS19, because they would then be vulnerable to a U.S. attack.

However, the Soviets instead appear to be adding another large silo-based, solid-fueled missile and a new mobile missile.