The United States yesterday reacted to the military coup in Turkey in a low-key manner that expressed hope for an "early restoration of democracy" but carefully did not criticize the action of the Turkish armed forces.

Underlying the reaction, which contrasted sharply with U.S. responses to other recent military takeovers in South Korea and Bolivia, was a clear feeling by U.S. officials that the Turkish situation is different and deserving of patience and understanding by Turkey's allies in the North Atlantic Treaty Organization.

Although no one would say so publicly, U.S. officials are known to believe that Turkey -- beset by political paralysis, crippling economic problems and mounting terrorism from extremists of the right and left -- had been staggering unavoidably toward some kind of internal upheaval.

Given that situation, U.S. officials seemed almost relieved that the interruption of democracy came not from such extremist forces as Turkey's communists or fundamentalist Moslems, but from the armed forces. They are regarded as moderate, pro-Western and committed to Turkey's role as NATO's strategically important southern anchor in the Mediterranean.

In fact, although U.S. officials insisted they had no prior knowledge that a coup actually was in the works, it did not come as a complete surprise. Since the beginning of the year, the Turkish military has issued several public warnings that it would step in to restore order if the country's feuding political parties failed to cooperate with each other in a concerted assault against terrorism and economic instability.

State Department spokesman John Trattner did reveal that the United States was advised of the coup one hour and 15 minutes before the military went on the radio to tell the country what was happening.

As Turkish military units were moving into downtown Ankara, Trattner said, a senior officer of the Turkish general staff telephoned the U.S. ambassador's senior military adviser to tell him that a coup was in progress and to give assurances that Turkey will stand by its international commitments, including its NATO membership, and that the action was not aimed against the United States or the 4,900 American servicemen in Turkey. a

Those assurances, repeated later in a statement to the Turkish nation by the chief of the general staff, Gen. Kenan Evren, were cited by U.S. officials as grounds for assuming that Turkey will remain firmly in the western camp and that the period of military rule will be of short duration.

With these factors in mind, Carter administration policy makers had Trattner read a carefully crafted statement that said in part:

"The United States must be concerned about the seizure of power from any democratically elected government. We note that in taking power the Turkish military have stated that they do so to restore a functioning democratic government."

Trattner then noted the problems with which Turkey has been struggling and pointed out that the United States, together with other Turkish allies, "has provided significant levels of assistance to help stabilize its economy and provide for the common defense.

"This assistance will continue," Trattner said. "We look forward to the early restoration of democracy in Turkey and to the establishment of economic and political stability."

U.S.-Turkish relations, which had been strained severely after Congress placed a partial embargo on arms sales to Turkey in response to the 1974 Turkish invasion to Cyprus, have improved greatly since two years ago when President Carter won a fight to have Congress lift the embargo.

U.S. aid to Turkey in the fiscal year ending this month involves $200 million for military sales credits and $200 million in economic assistance. For the 1981 fiscal year beginning Oct. 1, U.S. aid plans, which Trattner said will be honored, call for $200 million in economic aid and $250 million for military sales credits.

Turkey is important to NATO because it gives the alliance a strong military presence on its southern and eastern flanks. The United States has maintained bases in Turkey since the 1950s, in large part for monitoring activities in the neighboring Soviet Union.

These listening-post activities were curtailed sharply during the period of U.S.-Turkish strain, but the two countries more recently have been working out agreements allowing a resumption of most U.S. monitoring functions.

The restrained response to the coup shown by Washington appeared to be echoed yesterday in most other NATO capitals and at NATO headquarters in Brussels. Although most NATO governments appeared to be saying as little as possible, they did indicate they were encouraged that the coup leaders are well-known in NATO circles and are highly respected for their pro-Western attitudes.