Key U.S. officials made clear in interviews here that their approval of the new European Monetary System (EMS), recently adopted in principle in Bremen, will be withheld until the details of the system are worked out.

German officials are anxious to have President Carter give what one described as "a blessing" to the EMS, but will settle for much more guarded language in the communique to be issued here today at the end of the seven-power summit.

They accept completely the reservations that were expressed to Finance Minister Hans Matthoefer by Treasury Secretary W. Michael Blumenthal and Undersecretary of Treasury for Monetary Affairs Anthony Solomon in a meeting Friday. Reports that Blumenthal had endorsed the plan were in error.

As adopted in Bremen, the participating European countries would coordinate their exchange rates vis-a-vis third countries. The scheme calls for each country to deposit 20 percent of its dollar and gold reserves in a new fund, plus an equivalent amount of its own currency.

If gold is valued at the market level of around $175 an ounce and that is one of the controversial elements of the plan - the 20 percent of the reserves would total around $26 billion, and the matching currency would be the same amount, for a total of more than $50 billion.

The exchange rate relationships, according to the Bremen communique, "would be at least as strict as the (Snake)." The "Snake" of European currencies now allows a margin of 2.25 percent around current rates.

The weaker members of the Common Market, notably the British, fear that if their currencies are tied in to what is tantamount to a fixed-rate relationship to the D-Mark, the German monetary unit, and other strong currencies, they will have to follow deflationary policies.

The United States sees not only the potentially deflationary aspect of the scheme cited by the British but an inflationary aspect as well. This could arise not only through a kind of back-door revaluation of gold, but also by the deposit of national currencies, paper money against which the countries could draw new European currency units.

American officials are openly astonished that after all of the Germans' frequently stated concern about the expansion of world liquidity, and their caution against issuing new amounts of special drawing rights, they are willing to back the EMS.

It is clear that the EMS has a long road of negotiation ahead of it. British opposition is political as well as economis. There remains a suspicion among some groups in Britain of all elements of European unity.

But the pressures for the EMS are strong. West German Chancellor Helmut Schmidt is convinced that "the zone of monetary stability" that he and French President Valery Giscard d'Estaing are seeking is essential to stabilize the D-mark, and regenerate business investment within Germany. And both men would like the political yield from having fathered a real monetary union after so many tries and failures.

In the end, if assured that it won't harm U.S. interests and is in accordance with the global rules of the International Monetary Fund, the United States won't object. And that's a long way from reality at the present time.