The Carter administration, rejecting suggestions that it is getting only lukewarm support from its allies, insisted yesterday that America's major defense partners have promised strong backing for U.S. economic measures against the Soviet Union.

State Department spokesman Hodding Carter said President Carter has assurances that the principal West European nations, Canada and Australia will not sell the Soviets grain to make up for the 17 million tons being withheld by the United States in retaliation for the Soviet invasion of Afghanistan.

In private, other administration officials went beyond the spokesman's statement to stress their belief that, despite some outwardly ambiguous comments from some European governments, the western allies are in strong agreement on the need for concerted action in confronting the Afghanistan situation.

The officials said this allied approach was in marked contrast to the early days of the parallel Iran crisis, when some U.S. actions caused considerable acrimony between Washington and Europe. In the case of Iran, they noted, harmony wasn't restored until Secretary of State Cyrus R. Vance met with alliance leaders in Europe.

The officials conceded that some aspects of a joint allied position toward Moscow, particularly in the area of denying the Soviets access to high-technology imports such as computers and machinery, are still being worked out.

But they insisted that the degree of cooperation so far has been remarkably high, and they predicted that further strides will be made next week, when Deputy Secretary of State Warren M. Christopher is to go to Brussels for discussions with members of the North Atlantic Treaty Organization (NATO).

That, the officials said, is the case despite persistent reports that such European leaders as French President Valery Giscard d'Estaing and West German Chancellor Helmut Schmidt, whose countries have a strong interest in good relations with Moscow, are dragging their feet in lining up behind Carter's call for a tough stance.

Hodding Carter opened his daily news briefing by volunteering a statement of U.S. pleasure at "the very positive response" of the Europeans and of Australia and Canada, both major grain-exporting nations, to the partial grain embargo against the Soviets.

As a result, he said, "The Soviet Union will be unable through purchases of grain from other countries to meet the major shortfall caused by the president's decision."

"The United States feels that strong allied backing in this area demonstrates western solidarity in the face of a direct Soviet threat to international security and will make clear to the Soviets that they cannot undertake aggression with impunity," he said.

However, Agriculture Department officials estimated yesterday that the Soviets will be able to make up 3 million to 3.5 million of the 17 million metric tons being cut off by the United States through purchases and transshipments by Moscow's East European allies.

A metric ton is 2,200 pounds.

The East European countries have had poor domestic grain crops lately, and have been importing significant amounts of grain. As a result, the agriculture officials said, although efforts will be made to monitor East European purchases and hold them to what these countries need for their own requirements, a certain amount of slippage through to the Soviets seems unavoidable.

Another potential problem was posed by Argentina, which sold the Soviet Union 238,000 tons of wheat and 1.6 million tons of corn in 1979. President Jorge Videla's military government, which has been on bad terms with Washington, announced yesterday that it will not support the U.S. grain embargo against Moscow.

At the State Department, Hodding Carter said it was doubtful that Argentina could do anything to make up the Soviet shortfall. The Argentines, he said, do not have adequate uncommitted grain reserves to increase their sales of the Soviets.

However, his comments were disputed privately by some State Department officials who said the administration has indications that Argentina might have as much as 12 million metric tons of excess grain for export.

The Argentines are expected to participate in a meeting of major grain-producing countries here Saturday, and these officials said a major effort will be made then and afterward to pressure Argentina to cooperate. The problem, the officials conceded, is that the Argentines might try to extract, as the price of their cooperation, an end of U.S. criticism of the Videla regime's alleged human rights abuses.

The main thrust of U.S. efforts to forge a united front on economic sanctions has been aimed at the major industrial nations of the western world. That, in turn, has caused considerable confusion and speculation about the degree to which West European governments will cooperate on measures that go beyond agricultural dealings with the Soviets.

So far, most speculation has centered on France, which for years has tried to nurture a special independent relationship with Moscow, and on West Germany, whose "eastern policy" of reconciliation with the communist bloc opened the way in 1970 to a decade of East-West detente moves.

Schmidt has said publicly that his government cannot abandon the "eastern policy," or "ostpolitik," as it is referred to in German.

Last Sunday, French Foreign Minister Jean Francois-Poncet addressed the Afghanistan crisis in language so mild and vague that it caused a flood of French press commentary about France's "soft" position on economic sanctions.

However, U.S. officials dealing with Europe said yesterday that such statements were dictated largely by the need of West European leaders to guard their flanks against domestic political problems and by a genuine concern, shared by the United States, that detente not be choked off irrevocably.

As Hodding Carter noted in response to questions about European attitudes, "We are in an alliance, not a dictatorship." The United States, he said, cannot impose its will on NATO in the same way that the Soviet Union does on the East European satellite members of the Warsaw Pact.

Privately, other administration officials said Schmidt, although he so far has stopped short of endorsing specific economic sanctions against Moscow, is regarded here as staunchly supportive of U.S. efforts to check future Soviet actions like the Afghanistan intervention.

These sources noted that Schmidt always has considered close cooperation with the United States the cornerstone of West German foreign policy. And, while he has a strong interest in detente, he never has been as emotionally wedded to the "ostpolitik" as was his predecessor, Willy Brandt.

In regard to France, traditionally the most independent of the European allies, the sources noted that Francois-Poncet very specifically said France would not take advantage of the commercial opportunities to fill the gap created by the cutoff of Soviet access to U.S. grain and technology.

In addition, the sources continued, on Wednesday the French government, after a telephone conversation between Giscard and Carter, substantially hardened its stand on Afghanistan with a statement warning that France's commitment to detente is not unconditional and could be affected by actions such as a prolonged Soviet occupation of Afghanistan.