American officials here charged today that the Soviet Union is trying to sabotage U.N. "declaration of principles" on southern Africa sponsored by U.S. Ambassador Andrew Young.

"There is no question," said a ranking American diplomat, "that the Soviet Union has gone to the Africans to persuade them not to go along with the declaration. And there is no doubt that the Russians are pressuring the African liberation movements to toughen the joint. African negotiating position."

The Americans said they did not know whether the Soviet drive against the declaration was linked to Moscow's rejection of U.S. arms limitation proposals.

They noted that Soviet diplomats began their intense lobbying yesterday, the morning after the breakdown in the Moscow talks, and the same day that Foreign Minister Andrei Gromyko made his public attack on the American arms proposal.

Western and African diplomats, however, pointed out that Moscow had logical reasons to oppose the declaration for its own sake. Young and the moderate Africans with whom he was working hoped to get it passed yesterday, the last day of Young's tenure as Security Council president.

This could have provided dramatic evidence of American-African cooperation on southern Africa at the very moment that Soviet President Nikolai Podgorny was touring that area, promoting Soviet support for liberation of blacks in white-ruled countries through armed struggle.

The heart of the declaration, which most U.N. members accept, calls on South Africa to abandon its apartheid system, grant independence to Namibia (Southwest Africa) and stop supporting the white minority government in Rhodesia. The disagreement has been over how strongly the declaration should commit the West to take action against South Africa if it rejects the U.N. terms.

As of Wednesday night, "we could taste a deal," one American said. After the Soviets set to work the next day, however, the Africans took a harder line on aspects of their proposal referring to the liberation movements.

Now, as a result of British resistance to any mention in the declaration of a possible limit on Western economic relations with South Africa, as well as the harder African line, American officials and others involved in the three-week negotiations expressed fears that agreement was fading.

The United States seemed to be falling back, trying to avoid a vote on four tough African resolutions that could, if it comes to a showdown, force the United States into vetos and damage the credibility of Young and the Carter administration's policy toward southern Africa.

The Security Council postponed further debate on the subject until Monday, in the hope that a weekend of negotiations might achieve a compromise.

American officials said they hope to avoid a showdown and adjourn the debate until later this spring if agreement on the declaration is no longer possible.