A secret appendix to the arms treaty signed Tuesday by President Reagan and Soviet leader Mikhail Gorbachev reveals that the United States has deployed dozens more medium-range nuclear missiles in Europe than it has previously acknowledged, U.S. officials said yesterday.

The 114-page treaty appendix, which the Reagan administration decided on Monday to withhold from the public without offering an explanation, also reveals that the Soviets currently have 15 percent fewer medium-range missiles than the Reagan administration has publicly stated in recent weeks.

Highly detailed information about the number of U.S. and Soviet missiles covered by the treaty eliminating intermediate-range nuclear forces (INF) appears in a "memorandum of understanding" exchanged between the United States and the Soviet Union. The memorandum, or appendix, is part of the official documents signed by the two leaders at a nationally televised White House ceremony Tuesday afternoon.

Listed in the document is every site in the United States, Europe, and the Soviet Union where the medium-range and shorter-range missiles covered by the pact are located, as well as every INF weapons production, assembly, repair and storage site.

The purpose of this unprecedented exchange of information by Washington and Moscow was to ensure that both sides understood in advance exactly how many missiles will be eliminated under the pact and which sites in each country will be subject to on-site inspection by the other side to verify treaty compliance.

The memorandum lists 11 sites in nine states that Soviet inspectors can visit over a 13-year period after the treaty takes effect. A plant in Middle River, Md., where the Martin Marietta Corp. has produced launchers for the Pershing IA and Pershing II missiles covered by the treaty, is among the sites.

The document also lists 12 sites in five West European countries where Soviet inspectors can periodically visit, as well as roughly 70 locations in the Soviet Union and Eastern Europe where U.S. inspectors can visit.

Key data in the classified document was disclosed yesterday morning by the Natural Resource Defense Council, an environmental group headquartered here that has monitored U.S. and Soviet military activities. The information was confirmed later by knowledgeable U.S. government officials.

The government's decision not to release the document was made at the request of Pentagon officials who argued that the disclosure could invite terrorist attacks on the U.S. military bases it identifies, according to senior U.S. officials.

But other U.S. officials, including Secretary of State George P. Shultz and the chief U.S. negotiator of the INF pact, Maynard W. Glitman, have argued that the terrorist threat is minimal because U.S. nuclear warheads are not typically stored with the weapons deployment sites listed.

Shultz and Glitman have protested the administration's decision, which was also opposed by the Soviets. Gennadi Gerasimov, chief spokesman of the Soviet foreign ministry, said yesterday he plans to publish the document in a Ministry of Foreign Affairs bulletin that he edits.

White House spokesman Marlin Fitzwater suggested that the document, which was prepared over the last several months and reflects the status of U.S. and Soviet weapons on Nov. 1, may have been temporarily withheld so U.S. allies could first be informed of its contents. "It is not a closed matter," he said.

Shultz told reporters in Brussels on Nov. 25 after agreeing on key aspects of the treaty in Moscow with Soviet Foreign Minister Eduard Shevardnadze that it would eliminate "about 350" U.S. medium-range missiles in Western Europe.

Shultz said the United States and its allies had agreed to forgo the remainder of the planned INF deployments in Europe -- roughly another 220 missiles -- after the INF treaty eliminating those weapons was signed this week.

A State Department fact sheet on the INF treaty released to reporters on Tuesday provided a higher, but still inaccurate, estimate. It said single-warhead weapons "capable of carrying . . . almost 400" warheads would be eliminated by the pact.

But the appendix lists 429 "deployed" U.S. medium-range missiles in Europe, including 120 Pershing II missiles and 309 ground-launched cruise missiles. A senior U.S. official, contacted last evening for an explanation of the discrepancy, called it an inadvertent oversight.

Twelve of the Pershing II missiles are previously undisclosed spares deployed at three sites in West Germany, U.S. officials said. Previously undisclosed cruise missiles are also listed at sites in Italy and Britain.

Independent weapons analysts said they were surprised to learn from the appendix that the United States has also deployed 109 cruise missile launchers in Europe, each capable of carrying four missiles. Official U.S. plans called for 116 such launchers to be deployed there by the end of next year. The appendix also lists 100 completed cruise missiles awaiting shipment to Europe from production plants in Titusville, Fla., and San Diego.

In a separate section on Soviet missile forces, the appendix lists 405 deployed Soviet SS20 missiles, 36 fewer than previously published U.S. estimates. It also lists 65 deployed Soviet SS4s, 47 fewer than the administration has previously reported.

U.S. officials said these new totals, which were supplied by the Soviets in recent weeks, had been judged reasonable by the U.S. intelligence community.

One official added that the administration knew of the lower SS4 number, which he said resulted from "furious" efforts by the Soviets to dismantle as many of the 30-year-old missiles as they could since last summer, perhaps to make it look as though the Soviets were giving up relatively fewer weapons under the provisions of the INF treaty.

But the official added that the administration saw "no reason to give the Soviets a publicity coup" by publicly revising its estimate of the SS4 deployments. The official added that such estimates are normally updated in public U.S. documents only once a year.

Elsewhere, the appendix indicated that the United States has 430 medium-range and shorter-range missiles in storage, while the Soviets have 859 such missiles, all of which will be eliminated under the agreement. U.S. officials said the number of stored Soviet missiles were "at the high end" of a range of estimates prepared by the U.S. intelligence community before the U.S.-Soviet exchange of data.

Earlier, some U.S. officials said the Soviets had stored many more SS20s than estimated by U.S. intelligence. But these reports were based on incorrect numbers furnished by the Soviets to the United States that the Soviets later modified, a senior U.S. official said yesterday.

William Arkin, a nuclear weapons analyst at the Institute for Policy Studies who collaborated with the Natural Resource Defense Council in collecting the information, criticized the administration for its decision to withhold the data.

"Because the data are so detailed, they add substantial credibility to the treaty, the negotiating process," and the elaborate inspection procedures adopted by the two sides to verify compliance, he said.