The United States today became the first country to name names at the Belgrade conference on East-West detente by accusing the Soviet Union and Czechoslovakia of censoring mail in violation of international postal agreements.

Chief U.S. delegate Arthur Goldberg told reporters after today's session that the United States planned to raise additional specific humanitarian problems "in greater and greater detail" during the coming weeks of the conference, called to review adherence to the 1975 Helsinki accords.

U.S. delegate Joyce Hughes, a law professor and civil rights activists, exportedly told a closed plenary session today that Communist barriers to the free exchange of letters and publications represented "a continuing pattern of disrespect" for the 1975 Helsinki declaration.

The declaration was signed by the United States, Canada, and 33 European countries including the Soviet Union and Czechoslovakia.

Hughes reportedly cited principle 10 of the delcaration, which states that "the participating states will fulfill in good faith their obligations under international law" and principle 9, which calls on governments to cooperate in the economic, scientific, social, cultural, and humanitarian fields.

"When an American friend is unable to obtain delivery of a subscription to the National Geographic magazine for a Soviet schoolboy or a copy of the World Almanac for a teacher in Czechoslovakia, the flow of information is choked, not widened," she said, according to interviewed sources.

"These are not hypothetical incidents. These are actual cases. These are facts and we intend to address them forthrightly and with candor because we believe that a thorough review demands such candor and straight talk."

The fact that the United States chose to open its dossier on alleged Eastern European human rights infringements by referring to the relatively minor case of mail censorship was taken by some neutral delegates as a sign that Washington wanted to keep the debate fairly low-key at this state.

Delegates believe that an unspoken agreement exists between Washington and Moscow not to make too much trouble in Belgrade while crucial arms limitation talks are under way in Geneva.

U.S. delegates, however, reject such a contention, saying they will refer to more serious cases of human rights violations in due course.

So far, Western criticism of Eastern European human rights violations, specific or otherwise, has not drawn much reaction from the Soviet bloc delegates. The general tone of their reply has been that it is wrong to concentrate on specific items of the Helsinki final act at the expense of the document as a whole.

If pressed on human rights, they claim that it is not just a matter of individual rights but social and economic rights as well.

East German delegate Ernst Krabatsch summed up this view, asking the session what the point was of talking about a "handful of individual cases when human rights affect millions."

Soviet delegates have concentrated on their own proposals for disarmament and have largely ignored criticism.

"It's as if two express trains were driving in opposite directions," was the comment of one senior delegate on the different tactics being pursued by the Soviet Union and the United States.

The low Soviet profile is taken by some observers as evidence that they regard the Belgrade conference as an irrelevance when set against more important East-West meetings devoted to disarmament in Geneva and Vienna.

This attitude contrasts with their energetic lobbying in favor of the European security conference, the fore-runner of this conference, when the idea was first proposed several years ago.

Now that the conference has been in session for a full week, it has become clear that the United States is taking the lead in raising cases of human rights violations. The Western allies have yet to single out specific examples, being content so far with outlining general principles.