The Soviet Union apparently has begun to deploy its long-expected new mobile intercontinental missiles, and U.S. officials are divided over how to respond.

The Soviets also are destroying some older, silo-based missiles, a move that some officials believe means Moscow may be preparing to stay within the limits of the SALT II arms control agreement.

These officials, including some top-ranking military men, also believe that the mobile missiles, if swapped for silo-based ones, amount to a less threatening nuclear array. Therefore, they want to encourage Moscow to stay within the SALT II limits and destroy old, silo-based ICBMs as the new weapons are deployed.

To help accomplish this, these officials suggest, the United States should move to extend the limits, since the agreement expires at the end of this year.

But other administration officials consider that the first of the mobile missiles being deployed, the single-warhead SS25, violates the SALT II terms. Deployment of it, and a larger, rail-mobile, 10-warhead, SS24 missile, they say, will be destabilizing and result in a new escalation of the nuclear arms race.

They want the United States to oppose the new weapons.

The new Soviet missiles are bound to come up as a subject in the early strategic nuclear arms discussions between the two nations, which began Tuesday in Geneva, as part of a negotiation of the SALT II limits.

In an interview earlier this month, Col. Gen. Nikolai Chervov, a member of the Soviet general staff and spokesman on arms control matters, said the United States and the Soviet Union have agreed to discuss the SALT II limits at Geneva.

The reported Soviet deployment puts additional pressure on the negotiators to continue some limit while they attempt to achieve reductions. In addition, if it takes no action, the United States will exceed another of the SALT II limits, permitting only 1,200 multiwarhead missiles on land and sea, when the submarine Alaska is scheduled to go on sea trials this fall with 24 missiles.

There are signs of deployment of the single-warhead SS25 at two Soviet missile fields, according to intelligence sources.

Concrete shelters for 24 single-warhead, mobile SS25s at the Yashkarola missile base and another 24 at the Yuyra field have been completed. At Yashkarola, where the Soviets have 60 SS13s in silos, there are signs that those silos are being reworked. One source said this may indicate that some SS25s will be put in SS13 silos. The Soviets have said the SS25 is a modernized version of the SS13; President Reagan reported to Congress last month that the SS25 was a new missile, barred by SALT II.

The Soviet Union has tested successfully the larger 10-warhead SS24 intercontinental ballistic missile from a railroad launcher. A drawing of the SS24 will be printed in the 1985 edition of the Pentagon's "Soviet Military Power" booklet, expected next month, sources said.

Late last summer, the Soviets began to destroy 18 silos that held old SS11 single-warhead ICBMs.

There is disagreement within the U.S. intelligence community over whether this indicates the Soviets intend to adhere to SALT II limits on missile numbers or are preparing for a fast buildup of single- and multiple-warhead mobile ICBMs.

The SALT II agreement, which was never ratified by the U.S. Senate, limits each nation to 820 land-based missiles with multiple warheads. The Soviets now have 818 such missiles, based in silos. Deployment of just three SS24s, for example, without comparable reduction of an older multiwarhead land-based ICBM, would put the Soviets over the SALT II limit.

There is no agreement in the administration on the implications of SS24 deployments. Last week, White House national security affairs adviser Robert C. McFarlane described the mobile multiwarhead missile as a new "destabilizing system."

In a speech to the Overseas Writers, McFarlane said the SS24s "are swift, carry a big payload, are mobile and are accurate."

Later, however, a key Reagan adviser on weapons and arms control privately contradicted McFarlane's analysis, saying the observed tests of the SS24 show it is not as accurate as the silo-based ICBMs and thus less threatening.

Slightly larger than the new U.S. MX intercontinental missile, the SS24 is the first large solid-fuel Soviet ICBM. Solid fuel permits missiles to be more ready for quick launch than earlier liquid-fueled rockets. All U.S. ICBMs are solid-fueled except for 30 old liquid-fueled Titan IIs, which are being retired.

Current estimates are that the SS24 will not be deployed until late 1986, according to sources, but some officials believe it may come sooner.

A few may go into silos, but most will be carried on rail launchers, which will be difficult to locate and count, sources said. Such a launcher has been seen, one source said, traveling at the end of a train.

Tunnels and covered sheds along the thousands of miles of Soviet railroad tracks could serve as storage and hiding places for the SS24, sources said.

"They are going to have the mobile system that President Jimmy Carter wanted in Nevada and Utah," one analyst said.

The single-warhead, SS25 also is solid-fuel and travels on a tractor-drawn road-mobile launcher. With completion of shelters at the two missile sites, sources said, substantial emplacement is expected within the next six months.

Testing of this missile, which has been more successful than the larger SS24, is described by sources as "almost completed."

The SS25-type garages being built at Yuyra are being placed near shelters that currently house SS20 intermediate-range mobile missiles.

The mixing of the two types of mobile missiles has begun to cause difficulties for U.S. intelligence, which is trying to keep track and count SS24s because they have intercontinental range and therefore relate to the SALT agreements.

Some U.S. analysts say they believe that the destruction of the "headworks" of old SS11 silos relates to Moscow's intention to reduce old missiles as new ones are deployed. They point to the Soviet record in destroying old SS7 and SS8 ICBMs as required by SALT I, and 10 Yankee strategic missile-firing submarines as new ones came into operation. According to the Congressional Research Service, an 11th Yankee will have to be removed from service this year when a new Typhoon sub with 20 missiles goes into service.

However, others say Moscow's actions just as easily may indicate an intention to push forward quickly with new mobile missile deployments, while confusing the United States on whether it will stay within the agreed ICBM limits.

He noted that the SS11 silos could be reworked to take SS24s.

A more pressing problem associated with both the SS24 and SS25 weapons is the difficulty U.S. intelligence agencies have had keeping track of Soviet mobile missiles, because they can be moved or camouflaged when the most accurate U.S. photographic satellites pass over Soviet territory on predictable orbits.

A new, maneuverable U.S. photo satellite is not expected to go into orbit until later this year or early next to allow U.S. intelligence specialists to track mobile missiles.