The United States will respond to Soviet President Leonid Brezhnev's Oct. 6 offer to reduce nuclear arms in Europe only after the NATO alliance decides on deployment of new medium-range missiles and an arms limitation proposal of its own, according to Carter administration sources.

"We must have it clear in mind what we are doing," an administration source said yesterday in a meeting with reporters, before "any more exploration" of the Soviet leader's proposal is made.

U.S. officials will present to the North Atlantic Council in Brussels today a deployment and disarmament package for NATO nuclear forces that has been worked out among allied nation officials over the past six months.

A final decision on the plan will be made at a NATO Council meeting in mid-December.

Its key proposals included:

Production and deployment beginning in 1983 and continuing for up to three years of up to 108 U.S. Pershing II missiles and 166 U.S. ground-launched cruise missile (GLCM) launchers. Each GLCM launcher would have four missiles.

The proposed hardware package would total 572 new nuclear warheads that would have ranges of 1,000 or more miles and thus could hit targets in the Soviet Union from NATO bases. The longest range NATO missile system now deployed goes only 400 miles.

An arms limitation proposal to the Soviets for European nuclear weapons which would begin by tying the number of new NATO medium-range launchers eventually deployed to the number of similar long-range missile systems deployed by the Soviets.

Limitation on other European systems such as bombers capable of carrying nuclear devices, submarines that carry nuclear missiles targeted on Europe and short-range nuclear artillery and missiles would come later.

If deployment of the new systems is approved, the United States -- with allied support -- would remove about 1,000 warheads from the current nuclear stockpile in Europe. Since 1968, the United States has maintained about 7,000 nuclear warheads in Europe for a variety of delivery systems ranging from bombs to missile warheads to artillery shells.

The proposal to remove 1,000 existing warheads was proposed to the NATO allies by U.S. officials in the past month after Brezhnev announced his own decision to reduce 20,000 Soviet troops and 1,000 tanks from East Germany and negotiate nuclear weapons limitations if NATO turned down the plan to deploy new U.S. medium-range missiles.

"Removal of the 1,000 warheads," the administration spokesman said yesterday, "was not a response to Brezhnev."

It was, he noted, a move that was first considered more than five years ago and made part of a 1974 proposal in negotiations aimed at reducing troop levels in Europe.

It was resurrected last month "to improve the political atmosphere" within the alliance since several NATO countries "find the reduction attractive." The reduction emphasizes that modernization "is not a buildup" of nuclear arms, he added.

The spokesman made it clear, however, that if the medium-range missile deployment fails to get alliance approval, the reduction in warheads probably would not take place.

Under the proposed deployment package, the Pershing II missiles would be stationed primarily in West Germany while the 116 GLCM launchers, with their longer range, would be based in England, the Netherlands, Belgium and Italy.

The administration was described as quite confident" the deployment and disarmament packages would be approved by the alliance. The spokesman added that "the situation was quite encouraging" that all the host countries would accept the weapons assigned to them.

Countries such as the Netherlands and Belgium have in the past raised opposition to deployment of new U.S. nuclear systems -- particularly in 1977 and 1978 when neutron warheads and artillery shells were programmed for the alliance. President Carter finally deferred production of those weapons in April 1978.

As they did in the neutron debacle, West German leaders reportedly have been insisting that at least one other continental NATO power -- such as the Dutch or the Belgians -- accept basing on their territory of the new U.S. medium range missiles.

Vocal Soviet opposition to the new weapons, mixing threats of new Russian nuclear systems with offers to reduce warheads, has not affected what the administration source yesterday termed "the emerging consensus" on the part of alliance countries to go ahead.

Once the program is approved, he added "from that point we can go forward with Soviet negotiations."