President Carter told the leaders of the 15-nation North Atlantic alliance today that the United States is "prepared to make a major effort" to maintain the West's military strength, "in the expectation that our allies will do the same."

Carter made no specific demands and acknowledged that "difficult economic conditions set practical limits" on what can be done, a qualifying remark that his European audience appreciated.

Yet the President - in the strongest language he has used thus far on the military balance of power - said, "The threat facing the allience has grown steadily in recent years." Soviet and Warsaw Pact forces in Central Europe, he said, "emphasize an offensive posture (and) are much stronger than needed for any defense purpose . . . The pace of the Pact's build-up continues undiminished," he added.

"Our first preference," he said, "was for an early agreement with the Soviets on mutual East-West troop reductions in Europe, but failig that our military strength must be maintained."

Carter added a new twist to calls for more allied participation in mutual defense, suggesting that NATO set up a system for measuring whether countries meet the pledges they make toward overall defense goals.

The President's 20-minute address - which roamed far a field from military affairs into questions of human rights and relations with Eastern Europe - was received enthusiastically by the allied representatives here.

A West German Foreign Ministry spokesman termed Carter's speech "the clearest and strongest that has been hard in the Atlantic Council in many years."

British, French and West German diplomats said the speech contributed to the relaxation of tensions within NATO and produced a new feeling of "confidence."

Today's meeting was the third summit session for Carter in three days. For Europeans his presence at a NATO meeting was viewed as especially important - "a symbol", as British Prime Minister James Callaghan put it, "of his determination to support NATO with all the power and influence of the United States."

"This was never in doubt," Callaghan added, "but it is reassuring to have him reaffirm it."

Carters speech was reassuring on several counts to European leaders who have been worried and puzzled over some of his earlier moves.

Several European leaders, for example, fear that the United States will turn the coming 35-nation Belgrade conference, designed to review implementation of the 1975 Helsinki agreement on security and cooperation in Europe, into a confrontation with the Soviet Union over the agreement's human-rights provisions.

Europeans say that they, too, are concerned over these rights in the East, but that they fear that the Carter White House has been too outspoken and could provoke retaliation from the Soviets. This could hurt their efforts to have better relations with the East and to keep open the flow of emigrants.

Today, Carter said that the United States approaches the Belgrade conference "in a spirit of cooperation, not of confrontation." He said: "We support a careful review of progress by all countries in implementing all parts of the final act of Helsinki," not just the part on human rights.

Carter also went far afield from traditional NATO speechmaking when he called for the West to "try and draw the nations of Eastern Europe into cooperative undertakings."

"Our aim is not to turn these nations of Eastern Europe against the Soviet Union but," he claimed, "to enlarge the opportunities for all European countries to work together in meeting the challenge of modern society."

Carter also pleased the Europeans by reaffirming that the United States supports a negotiated East-West troop reduction in central Europe based on parity in the numbers of front-line troops.

The President also said the United States must be willing to promote genuinesly "two-way trans-Atlantic trade" in the arms-producing field. He gave no specifics, but Europeans who have complained for years of a U.S. in arms production were glad to hear the U.S. president say this.

Carter called for cooperation in the development of new weapon through the newly formed European Program Group, which is on the fringes of NATO. This is significant since it would allow France - which withdrew from NATO itself - to participate.

In pleeging to keep NATO as "the heart of our foreign policy," and also to strengthen political and economic cooperation, Carter suggested:

A special review of East-West relations including assessments of future trends in the Soviet Union and weastern Europe and their potential implications for the West.

A high-level study aimed at coming up with a better long-term defense program for NATO.

Carter's aides said the President was not suggesting the need for any big increase in defense spending, but an effort to use the funds alreay budgeted with less waste and more efficiency.

Carter also held a luncheon meeting with the leaders of Greece and Turkey and later said both were determined to solve their differences in an amicable way. A senior aide to the President said privately, however, that there was little ground for optimism.

West German Chancellor Helmut Schmidt also delivered a major speech here today in which he praised carters remarks and pledged that the West Germans would continue to do their share in NATO, but cautioned that the greatest danger tht the West faced today lay in the economic and monetary field.

Carter told reporters tonight that he felt the "common sharing of feeling toward NATO in the U.S. - which was endangered a few years ago - has been restored."