U.S. intelligence agencies may have underestimated the number of 300-mile-range SS23 nuclear-armed missiles deployed by the Soviets in Eastern Europe and totally missed the placement of Soviet ground-launched cruise missiles in Latvia if data provided by Moscow as part of last week's Intermediate-Range Nuclear Forces (INF) Treaty proves correct, according to government and congressional sources who have analyzed the information.

On the other hand, the analysts said the data indicate that the Pentagon's Defense Intelligence Agency (DIA) for several years may have overestimated by more than 30 percent the overall size of the Soviet intermediate-range nuclear SS20 mobile missile force.

New and sometimes unexpected information on Soviet nuclear weaponry was contained in the "memorandum of understanding," signed by Soviet leader Mikhail Gorbachev and President Reagan, published as part of the treaty and released to the public last week.

It has since caused the DIA and the Central Intelligence Agency to review their past estimates, according to informed sources.

The data also have put new life in longstanding intelligence community disputes and promises to provide fuel to congressional critics of the treaty who argue the Soviets cannot be trusted.

After the INF Treaty takes effect, the Soviets will provide an updated data base and U.S. teams will carry out on-site inspections of listed facilities to verify the data.

Until early this year, according to sources, U.S. intelligence could confirm only that about 10 SS23 missiles had been deployed at a base in the western Soviet Union. Then, according to these sources, a second base was discovered by chance by a U.S. military attache in East Germany.

But last week, the Soviets reported a total of 82 SS23 launchers and 167 missiles deployed in East Germany and the western Soviet Union. Sources said two Soviet bases with nuclear missiles were unknown to the United States. "That just proves smaller missiles are tougher to find," a congressional intelligence expert said.

Soviet disclosures about its SSCX4 ground-launched cruise missile, which was not projected for deployment until 1988 or 1989, have caused the biggest stir in U.S. intelligence circles.

The Soviets said last week they had 84 of the 1,800-mile range missiles "in storage," and six launchers in a site near Jelgava, south of Riga, capital of Latvia. They said "elimination facilities" are at the same site. The Soviets describe their cruise missiles as "tested but not deployed."

One source said moving the weapons to Jelgava showed how easy it may be to hide a usable nuclear force. Another said it was likely the Soviets made the move from the Kapustin Yar test range south of Moscow to Latvia to keep U.S. inspection teams away from the test facility.

The SS20 controversy shows how the United States tried to deal with a missile never seen by reconnaissance satellites. In 1975, before the first SS20 was built, sources said, a U.S. spy in the Soviet Union said Moscow planned five missiles for each launcher. Caught and executed in 1979, the spy's early information guided subsequent U.S. estimates.

SS20 deployment was cloaked in secrecy; missile, transporter vehicle, and protective transport cannister were never seen by the United States, which estimated the number of SS20s in the field by counting the garages for launchers and transporters.

"We counted garages, which we could see, not missiles," one intelligence sources said.

As the number of garages increased above U.S. projections, the CIA lowered its estimate of the "refire" SS20 force; DIA did not. Pentagon and DIA officials continued to insist on at least one extra "refire" SS20 for every deployed missile.

The Defense Department's 1985 edition of the unclassified "Soviet Miltary Power" estimates "about 400 deployed {SS20} launchers -- with a . . . 3-warhead missile and reload." The 1987 edition indicated up to 900 SS20s in all, to go with the 441 missiles DIA reported as having been deployed.

This year's edition said that "the SS20 launcher can be reloaded and refired, and the Soviets stockpile refire missiles."

Last week, the Soviets reported having 659 SS20s, deployed or in some form of storage.

They said 405 of the mobile SS20s along with their launchers were deployed at 39 bases, 15 missiles with launchers were stored at 10 of those same sites, and 164 SS20 missiles were at two storage facilities.

In addition, 36 missiles were at the production facility and 29 missiles and 68 launchers were at an "elimination facility."

Senate critics of the INF Treaty have focused on the DIA numbers and charged, according to a congressional analysis done for one Republican legislator, "the Soviets may secretly be able to retain at least 250 more {SS20s}."

Treaty supporters said the Soviet numbers are within the range estimated by the CIA, and that the DIA estimates were wrong.