The Soviet Union has indicated a readiness to make substantial concessions on an agenda for East-West talks on detente that could lead to a searching review of its human rights record by Western countries.

According to Western sources, Soviet chief delegate Yuli Vorontsov has signaled that he is prepared to accept guidelines drawn up by natural countries for a higher-level meeting in the fall. The aim of the conference is to examine progress since the 1975 Helsinki declaration signed by 33 European countries, the United States and Canada.

"This could be the breakthrough we have been waiting for," a senior Western delegate said as the preparatory phase of the talks entered its fifth week.

Other delegates, however, cautioned against assuming that formal agreement on the shape of the meeting will necessarily follow swiftly.A number of obstacles remain, the trickiest being conflicting views on whether there should be a firm cut-off date to the conference.

If the neutral's proposal is accepted, powerful committee would be established to check compliance with each part of the Helsinki final act and to discuss ways of strengthening detente in the future.

A separate committee would be established to consider progress, or lack of progress, on the human rights pledges made at Helsinki. The participating states promised, among other things, to respect freedom of speech, freedom of movement and freedom of the press.

Other committees would examine security in Europe, economic cooperation and contributions by non-participating Mediterranean states to the conference.

The Soviet Union had earlier made clear that it wanted the main work of the conference carried out in plenary sessions rather than committees - a proposal that Western delegates claimed would lead to a "mammoth talking shop."

Under the neutrals' proposal, the committees would begin work in the third week of the main conference, after two weeks of plenary sessions designed to initiate the debate.

Members of the U.S. delegation confirmed that the present talks have entered a more positive phase but denied that any definitive agreement is imminent.

"We have the feeling that the logjam is beginning to move, but a lot still remains to be achieved," one delegate commented.

The Soviet Union still has not dropped its demand for a fixed time limit to the talks. This has been resisted by Western delegations, particularly the United States, on the ground that it could enable the Soviet bloc to organize a giant filibuster.

The main reason for the new air of optimism at the Belgrade talks, at which little progress has been made until this week, stems from an apparent change in the Soviet attitude toward the agenda and organizational framework of the main meeting.

News of the Soviet change of heart was given by U.S. Ambassador Albert Scherer to other senior delegates at a luncheon here today. It followed the circulation of a detailed proposal by neutral countries that effectively insisted on a full review of the implementation of the Helsinki declaration.

American delegates are unhappy about a number of ambiguities in the neutrals' draft agenda that could still create difficulties. The interpretation put on this draft by Western countries could differ substantially from that of the Soviets.

"We need time to consider the implications of what has been happening here over the last few days. Most delegations will be consulting their respective capitals," said a West European delegate.

According to unconfirmed reports circulating at the conference center. Vorontsov will soon be recalled to Moscow for consultations with Kremlin leaders.Such talks could herald an entirely new phrase of the conference.

So far, Vorontsov's tactics have been conciliatory one day and threatening the next - another reason why American delegates are reluctant to talk about a spectacular breakthrough yet. There is still plenty of opportunity for him to be stubborn.

There is also considerable speculation over the reasons for Vorontsov's new spirit of compromise. It could be a tactical device to dispel the sense of isolation surroudning the Soviet-bloc delegations following presentation of the neutrals' proposal.

By indicating a readiness to concede on the agenda and committees, the Soviet Union may be preparing to dig in its heels on the question of the cut-off date.

A number of subsidiary issues could also ensure that the present meeting goes its full length until the end of this month. The principle that all decisions are made on the basis of consensus means that small countries possess considerable disruptive power.

Malta, for example, has already indicated that it intends to press for increased discussion of Mediterranean problems. The Maltese want countries that border the mediterranean to participate in the committee stage as well as n the plenary.

Maverick Romania, too, sees the conference as a way of increasing its independence from other Warsaw Pact nations. Western delegates believe that it could provide delays by insisting on an institutionalized followup to this year's meetings.