As one final element in SALT II, the strategic arms limitation treaty, the United States is pressing the Soviet Union to agree to limit the number of warheads carried by any individual intercontinental ballistic missile according to informed sources.

Both countries have already agreed to limit to 1,200 the number of submarine and land-based ICBMs that are able to carry more than one independently targeted warhead.

But U.S. officials fear that even with the limitation on these missiles with multiple independently targeted warheads, known in strategic weapons parlance as MIRV's, the arms race could continue in the future as each side put more warheads on the missiles they were allowed to have.

The U.S. negotiators, therefore, have proposed that each country be prohibited from increasing the number of warheads on the MIRVed missiles that are now deployed.

In addition they are pressing for limit on the number of warheads, said to be 10 to 14, that can be placed on any new ICBM.

Although this is one of the items on which final agreement has yet to be reached, U.S. officials do not see it as one of those now holding up conclusion of the SALT II negotiations.

Lacking some limitation, the United States has a great deal to lose in a race to add warheads. The Soviet land-based missiles for the most part can carry much larger payloads than the smaller U.S. ICBMs. This advantage in what analysts call throw-weight would allow the Soviets to add greater numbers of warheads to their MIRV missiles if they so desired.

For example, the Soviets currently are deploying a missile, the SS18, that reportedly has five to eight independently targeted warheads. It replaces an older missile that had only three.

Because of the large size of the SS18 rocket engine, it could be modified in the future to carry 20, 30 or even 40 warheads that are smaller than the five to eight it now carries but still powerful, U.S. defense experts say.

SALT II critics in the United States have already questioned the draft agreement because it calls for equal numbers of MIRV missiles even though the Soviets have greater throw-weight in their missile systems.

The United States, on the other hand, has a slight advantage over the Soviets in numbers of ICBM warheads and advantage is scheduled to grow as the Trident sub-based missile, with its eight to 10 warheads, come into the force to replace the Polaris and some land-based ICBMSs that have three or fewer warheads.

Currently, for example, the United States has about 4,500 land-based and sub-based ICBM warheads while the Soviets reportedly have a somewhat smaller number.

In the 1980s, however, the U.S. ICBM warhead total - given SALT II missile limitations - is programmed to climb to about 8,700 while the Soviets figure - under the same SALT II missile limitations - would be at about 7,000.

The warhead limitation reportedly now under discussion with the Soviets would freeze each side to the number of warheads already deployed on each of the present missile systems. This would give the United States an advantage to match the Soviet's throw-weight.

Along with maintaining a numeral warhead advantage, the warhead limitation could help one proposed strategy for keeping the U.S. land-based ICBMs from being vulnerable to a Soviet first strike.

Under the so-called multiple aim point concept, termed MAP, a mobile U.S. ICBM would play a kind of shell game - traveling among 10 or more holes so the Soviet planners wouldn't know which hole to destroy.

However, if the Soviets could keep addind warheads to their MIRV missiles, the United States would have to keep adding holes to keep ahead.

Along with freezing the now-deployed missiles to their current warhead numbers, the United States is also seeking to limit the number of warheads that can be put on the one new land-based ICBM system each side is being permitted to develop.

U.S. nuclear scientists at work on the proposed MX land-nased ICBM have explored the possibility of putting up to 19 warheads on a single missile. Although current planning does not call for that many warheads to be on any final MX design, scientists believe with some technolgical advances, even more than 19 warheads would not be impossible in the future. The Soviets, they fead, are not far behind.

Continuous technological advancement, both in the accuracy of long-range missiles and in the amount of nuclear materials needed for given levels of explosive power, have already enabled bothe the Soviet Union and the United States to make their warheads smaller and smaller.

Thus, although the warhead limitation has had almost no publicity in the SALT discussions, Carter aides see it as one of the more significant arms control elements in the current negotiations - if agreement can be reached.