Soviet Communist Party leader Leonid Brezhnev declared tonight that the threat of a new nuclear arms race is increasing because the Carter administration seeks "to obtain unilateral advantages" in talks on curbing strategic arms.

His remarks, broadcast on Soviet television, differed markedly in tone from President Carter's generally optimistic appraisal of the Strategic Arms Limitation Talks (SALT) following the last round of negotiations between Secretary of State Cyrus Vance and Soviet Foreign Minister Andrei Gromyko in Geneva earlier this month.

The Geneva meeting, Brezhnev said, "did show signs" of progress "on some of the issues . . . but I must say frankly, no serious forward movement has so far been achieved in view of the unconstructive line of the U.S.A."

Soviet "concern about the continued arms race," he said, "has grown because of the line taken by the new American administration. It is patently geared to obtain unilateral advantages for the U.S.A. Quite naturally such a line does not assist the preparation and conclusion of a new long-term agreement . . . which has already been delayed too long as it is."

Brezhnev's pessimistic comments were essentially the same as Gromyko's before he left Geneva last week-end. But they gain considerable additional importance coming directly from the party leader - and particularly after Carter's own description of the talks as "upbeat."

What the Soviets seem to be asserting is that despite an agreement at Geneva on a "framework" for futher discussions - essentially an agenda - Kremlin still has fundamental objections to the American approach on such key matters as the role of the U.S. cruise missile.

"The most important thing of all," said Brezhnev, "is that the American administration should take up a fully realistic stand and proceed from the principle of equality and equal security."

His comments on SALT came in an informal speech carried on French and Soviet television as part of the build-up to Brezhnev's visit to Paris next month, his first such trip to the West in nearly two years.

The Soviet leader also appeared to be rebuking Carter and other Western leaders who have indicated that the Belgrade conference to review implementation of the 1975 Helsinki accord on European security should be, among other things, a forum for judging the Soviet record on human rights.

Brezhnev said the meeting, which begins in Belgrade June 15, can benefit detente it "held as an undertaking of cooperation and not of arguement. Anybody who should try to orient it in a different direction would be assuming a great responsibility."