The Carter administration confirmed yesterday that revised intelligence assessments have come to the "unambiguous conclusion" that the Soviet Union has a force of 2,000 to 3,000 combat troops in Cuba.

State Department spokesman Hodding Carter, in providing that confirmation, said the combat force "as currently configured and supported poses no threat to the United States."

But Carter said the United States is concerned about the Soviet presence on the island and has raised the issue with Soviet representatives.

President Carter discussed the situation by phone with Secretary of State Cyrus R. Vance yesterday from Plains, Ga., but referred all questions to the State Department.

Meanwhile, a high-level Cuban official yesterday described the reports as "a deliberate falsehood" designed to slander Cuba while it is hosting a conference of nonaligned nations. (Story, Page A14.)

Soviet ground forces "per se did not figure in our bilateral understandings with the Soviets" that came out of the Cuban missile crisis of 1962, "which were directed toward offensive weapons systems," Hodding Carter said. He added that "we have no evidence that the Soviets have misled us on numbers or anything."

The State Department spokesman did remind reporters, however, that President Carter has said he will oppose any direct or indirect efforts to establish military bases in the Western Hemisphere.

Yesterday's acknowledgment by the Carter administration that it may have overlooked as many as 3,000 Soviet combat troops in Cuba stunned some intelligence executives who believe the number will end up much lower than that.

One theory is that there may have been "doublecounting," as intelligence analysts counted advisers as combat troops in this new estimate.

The heart of the U.S. government's concern, officials said is not about the number of Soviet troops in Cuba but about their mission. Intelligence leaders flatly admitted yesterday they do not know what that mission is, but that they are mobilizing their resources to find out.

Those resources include a limited amount of information from agents, some electronic intercepts, and photographs taken by spy satellites and reconnaissance airplanes, when the president authorizes overflights.

Since 1976, officials said, the United States has credited the Soviets with having a force of about 2,000 military advisers in Cuba to train Cuban troops to use Russian equipment ranging from armor to Mig 23 fighter planes.

In addition to those technical advisers, U.S. intelligence has detected since 1976 some combat units that did not appear organized into anything as cohesive as an American-style brigade.

But this summer an argument arose within the U.S. intelligence community as to whether the Soviets were in the process of putting together a brigade. Soviet military clerks and other headquarters types who would provide the nucleus for a brigade were detected going into Cuba.

This command structure was not fleshed out with troops, prompting administration leaders to reassure Congress as recently as July that there was no solid evidence of any big, new combat brigade shaping up in Cuba.

Secretary of State Cyrus R. Vance, Defense Secretary Harold Brown and Gen. David C. Jones, chairman of the Joint Chiefs of Staff, all took that view.

"There is no evidence," Vance wrote Sen. Richard Stone (D-Fla.) July 27, "of any substantial increase of the Soviet military presence in Cuba over the past several years or of the presence of a Soviet military base.

"Apart from a military group that has been advising the Cuban Armed Forces for 15 years or more, our intelligence does not warrant the conclusion that there are any other significant Soviet forces in Cuba."

At a closed hearing of the Senate Foreign Relations Committee July 17, Defense Secretary Brown and the nation's top intelligence executives gave that same kind of report.

Gen. Eugene F. Tighe, Jr., director of the Pentagon's Defense Intelligence Agency, and Adm. B. R. Inman, director of the National Security Agency, were with Brown when he testified at the secret hearing.

In contrast to that united front presented at the top by Carter administration officials a secret battle was raging behind the scenes at the lower levels of the intelligence agencies.

Some specialists, apparently on the basis of electronic intercepts, insisted there was something new going on among the Soviet military forces in Cuba. The Defense Intelligence Agency hierarchy, sources said, discounted that interpretations.

"I know some of them knew," said retired Lt. Gen. Daniel Graham, former head of the DIA, referring to lower-echelon DIA officials and their interpretation of increased Soviet military presence in Cuba. "But I can't say that the people who knew were able to convince their colleagues."

The dispute heated up to the point that the Central Intelligence Agency, which directs the spy satellite program, ordered intensive photographing of Cuba. The resulting photos startled many top officials.

U.S. satellites peering down on the island snapped pictures within the last two weeks of tents spread across the Cuban landscape and of armored vehicles and artillery. Soviet troops, it appeared beyond any reasonable doubt, were out in force on military maneuvers, not just showing Cubans how to shoot Russian weapons.

Analysts pored over the satellite photos and estimated how many troops were involved in the combat exercises and how many others were out of sight of the cameras. The Soviet combat troops appear to be based near Los Palacios on the western end of Cuba.

Yesterday, using figures still under challenge in some parts of the intelligence bureaucracy, the Carter administration put an entirely new face on the Soviet military presence in Cuba.

Said State Department spokesman Carter:

"We have recently confirmed the presence in Cuba of what appears to be a Soviet combat unit. This is the first time we have been able to confirm the presence of a Soviet ground forces unit on the island.

"Elements of the unit appear to have been there since at least 1976. We estimate that it consists of 2,000 to 3,000 men. The unit includes armored artillery and infantry elements. In addition, we estimate that [the Soviets] maintain between 15,00 and 2,000 military advisory and technical personnel in Cuba."

Intelligence officials, asked how up to 3,000 combat troops could go undetected for so long, said they receive little good information from within Cuba, and that overflights by spy planes such as the SR71 Blackbird were suspended by President Carter right after he took office. (Carter made an exception late last year to see if Soviet Mig 23 fighters in Cuba were dequipped to carry nuclear weapons. The administration said they were not.)

Intelligence officials also said that so many ships and planes go into Cuba these days it is hard to keep track of what they carry. The Soviet combat troops went into Cuba gradually over the last three years, they said.

Asked at a press conference yesterday whether the United States had suffered another "intelligence gap" by failing to detect the Soviet combat troops sooner, Senator Stone replied there had been an "intelligence insufficienty" at the "gathering level."

Asked if Brown had misrepresented the situation in Cuba to Congress, Stone replied: "He was misadvised,"

Stone said he was alerted to the Soviet troop increase by a staffer who had read an intelligence report. "There is more to the Soviet presence than has been described," Stone said, refusing to elaborate.

Like administration officials, Stone said he does not know what the Soviets have in mind for their combat brigade. But the senator contended the brigade represents a military base that must be removed because it violates the Monroe Doctrine.