Pact on long-range missiles unlikely, US officials say. Soviet and US agreement on comprehensive arms control is a long way off, administration officials say. The sides show no signs of coming together on SDI, for example. For now, the greatest hope is seen for an accord on intermediate-range missiles.

Negotiators from the United States and the Soviet Union have rolled up their sleeves and sat down to serious work on a draft agreement to slash strategic nuclear arsenals in half, say administration officials. But chances are still remote that such a historic pact will actually be signed by President Reagan and Soviet leader Mikhail Gorbachev, according to knowledgeable sources. Little progress, if any, has been made on key stumbling blocks. These include:

The Strategic Defense Initiative (SDI). The Soviet Union continues to link reductions in long-range strategic arms with limits on the US SDI program. Soviet negotiators have in recent weeks hinted that they would accept less stringent restrictions than those they had previously sought, but the US still says it won't accept any SDI limits.

Mobile missiles. The US wants to ban mobile intercontinental ballistic missiles (ICBMs). The Soviet Union has resisted this view and in recent months has deployed a new rail-mobile ICBM, the SS-24.

Sublimits. The two sides have not yet seriously negotiated limits for particular weapon types, such as heavy ICBMs. Although the US has pressed this issue, the Soviets have proposed only general limits in recent months.

Overall, ``there has been no change on the major issues,'' says a US official involved in the arms control process.

Meanwhile, much progress has been made toward an arms pact that would eliminate another, less militarily important type of nuclear arm: intermediate-range missiles (INF). Says a second administration official: ``My own personal view is that the Soviets think an INF pact will do it'' as far as arms treaties with President Reagan.

There has been movement in the Geneva talks on strategic arms, insofar as political posturing seems to be over and both sides have settled down to professional negotiation.

Two sets of strategic talks are ongoing. One set, the strategic arms reduction talks (START), focuses on strategic offensive arms. A second deals with space and strategic defense, and focuses on possible modifications to or extension of the 1972 Antiballistic Missile (ABM) Treaty. Soviet negotiators say a START treaty cannot be signed unless there is also agreement in the second set of talks on space.

In late July, the Soviet Union submitted draft treaties in both these talks. On strategic offensive arms, the two sides are now drawing up a joint draft treaty listing points of agreement and disagreement.

Sublimits and mobile missiles are two areas where obvious disagreement will be listed. But even if these other difficult obstacles can be eliminated, there will still be what is widely accepted as the major difference between the two nations' negotiating positions: SDI.

``You could get fundamental agreement on START but it could still be held hostage to SDI,'' said John Tower, a former Reagan administration arms negotiator and US senator from Texas, at a recent breakfast with reporters.

US officials say there are hints that the Soviets have some flexibility in their position in the space defense talks. Soviet negotiators have called for limiting SDI to laboratory tests. But they appear willing to accept a fairly loose definition of laboratory, to include tests run outside on the ground. US officials claim this restriction would still gut SDI.

A number of administration sources say they do not interpret Soviet treaty language as giving up on the past Soviet demand that it have a veto over deployment of SDI in any strategic agreement.

The question of whether to give a little on space defense is a divisive one in the administration, officials admit. Paul Nitze, chief arms control adviser to the State Department, has hinted publicly that he believes limits on SDI testing in exchange for large cuts in strategic arms could be a good deal.

But the real power centers in setting administration arms control policy - the National Security Council and the Defense Department - have shown no signs of giving in on their stance that SDI is absolutely not up for negotiation, a number of Washington analysts say.