U.S. government sources said yesterday that the apparent overthrow of Hafizullah-Amin's Afghanistan regime appears to have resulted from a Soviet military intervention almost the equal of Moscow's 1968 invasion of Czechoslovakia.

While conceding that their information was still very fragmentary, the sources said there was no doubt that some of the Soviet combat forces carried into Afghanistan by a massive airlift during recent days took part in the fighting in Kabul yesterday.

In addition, the sources added, the apparent installation of former deputy prime minister Babrek Karmel as Afghanistan's new leader seems aimed at putting in place a "puppet" even more loyal to Moscow than Amin's basically pro-Soviet regime.

The military move against a neighboring country -- the most blatant display of Soviet intervention since it invaded Czechoslovakia to displace what it regarded as an unreliable regime -- is certain to affect U.S.-Soviet relations across a broad range of issues.

For one thing, it comes at a time when the United States is engaged in a tense confrontation next door with Iran and has a strong interest in not intensifying the already strong stirrings of turmoil in the Persian Gulf and contiguous areas in the Middle East and Southwest Asia.

In addition, a Soviet display of undisguised military power such as that staged in Kabul yesterday is bound to increase concern in the United States about the degree to which Moscow is committed to detente. One immediate casualty could be the already weakened hope of the Senate approving the strategic arms limitation treaty, and beyond that is a possibility of escalating U.S. Soviet strains in almost all other areas of potential cooperation.

For these reasons, U.S. sources, while saying the reasons for Moscow resorting to such tactics were not immediately clear, said the Soviet leadership obviously feels it has a great deal at stake in Afghanistan.

According to the sources, the strike against Amin seems to represent a major shift in the Soviet involvement with Afghanistan.

Since 1978 when a coup brought a coalition of Marxist forces to power in Kabul the Soviets had been increasing their influence, to the point where U.S. officials expressed concern about the country being transformed from an autonomous Southwest Asia buffer state to a client of Moscow.

The increasing closeness to the Soviets caused corresponding strains in Afghanistan's relations with the United States. These relations reached a very low point early this year when antigovernment rebels kidnaped U.S. Ambassador Adolph Dubs, and he subsequently was killed when Afghan police, directed by Soviet advisers, attacked the hotel room where he was being held captive.

But until yesterday, Soviet policy had seemed aimed at bolstering Amin and his supporters against threats from rebel Islamic tribesmen who have turned much of Afghanistan's countryside into an arena of civil war.

In fact, when the State Department revealed on Wednesday that the Soviets were airlifting large numbers of combat troops into Kabul and massing five divisions on the border with Afghanistan, the initial assumption of U.S. analysts was that the buildup was intended to assist Amin in some way with his struggle to subdue the rebels.

Instead, the sources said, it now appears that the airlift was a Trojan horse designed to get Soviet troops into the country so that they would be in a position to turn on the unsuspecting Amin.

That, the sources continued, seems to mean that Moscow had lost faith in Amin's ability to reverse his regime's deteriorating position and build a support base capable of controlling the country.

Amin, who ousted the original 1978 coup leader, Nur Mohammed, Taraki, three months ago, was regarded as a dedicated Marxist. But, the sources noted, he also had a reputation for rigidity and a sense of nationalistic independence that apparently led Moscow to conclude that someone more docile was needed.

Robert G. Neumann, who was U.S. ambassador to Kabul from 1966 to 1973, said yesterday that Amin "had so alienated the country that the Soviets essentially were left with three choices. They could let Amin fall, they could move in immediately with masses of troops, or they could try to find a political solution."

"It appears that they opted for a combination of choices two and three," concluded Neumann, who is now at the Georgetown University Center for Strategic and International Studies.

He agreed with Carter administration officials, who declined to be identified, that the Soviets will now try to use Karmel to woo the dissident forces in Afghanistan into greater support for the central government.

Neumann, who said he knows Karmel well, characterized him as "a dedicated communist (who) always follows the Moscow party line" and predicted he would carry out Moscow's wishes "to the letter."

Karmel headed a faction in the group that took power in 1978 that lost a power struggle with Amin's forces and was sent into exile. Prior to his apparent surfacing in Kabul yesterday he had been living in Prague, and U.S. sources said it seemed highly unlikely that he could have gone back to Afghanistan unless he was taken there by the Soviets.

Several close associates who share his loyalty to Moscow also have been living in Eastern Europe during their exile, and Neumann predicted that they too will resurface shortly in a new Moscow-directed regime. He characterized them as "a spare wheel" that the Soviets were able to call upon when they decided a change was needed.

The use of Soviet troops to overthrow a government in a neighboring country outside of Europe appears to open an entirely new stage in the postwar history of Soviet intervention and expansionism, the sources noted.

In the first stage, the Soviets capitalized on their World War II victory against Germany to set up a string of loyal satellite states in Eastern Europe. To keep this buffer intact, they periodically have intervened over the years in these countries -- putting down an uprising in East Berlin in 1953 and later staging massive nation wide invasions of Hungary and Czechoslovakia.

They also put sizable forces in Cuba after that Caribbean country moved into the communist orbit in 1959. But in regions outside of Eastern Europe, the Soviets, until now, have limited their military activities to providing arms, equipment and advisers to countries seeking aid from Moscow.

More recently, when opportunities for communist military intervention have come up in such Third World areas as Africa, the Soviets have turned to their Cuban ally to provide surrogates. That resulted in large-scale Cuban forces being sent with Soviet complicity to such strife- torn African countries as Angola and Ethiopia.

But, until two weeks ago when U.S. officials first began to detect the movement into Afghanistan of Soviet combat forces now estimated to total at least 6,000 men, the Soviets have steered clear of putting their own troops on the fighting line in areas outside of Eastern Europe.