President Carter, attempting to seize the political initiative in the wake of another deadlock in Strategic Arms Limitations Talks, said yesterday he will "hang tough" in pressing the Soviet Union to accept one of the American proposals that were rejected in Moscow.

However, the President insisted that he was neither surprised nor discouraged that the initial attempt to reach agreement on sweeping reductions in nuclear arms had failed. Moreover, he added, he found it "very encouraging" that the Soviets agreed to continue the negotiations with a new round of talks scheduled for May in Geneva.

Carter spoke to reporters, his manner relaxed and confident, during an impromptu press conference at the White House.

The President said the Russians insisted that the Nixon and Ford administrations had agreed not to deploy the American cruise missile. This, he said, was the major stumbling block preventing agreement on the more modest of the two U.S. proposals - to have the two countries simply ratify the nuclear arms levels set in the 1974 Vladivostok agreement.

Carter said he rejects this contention - and is supported in this by both former President Ford and former Secretary of State Henry A. Kissinger - and will continue to seek agreement on the main American proposal, which calls for "substantial reductions" in strategic nuclear weapons, a ban on deployment of new nuclear weapons systems and a limit on missile test firings.

But in response to a question, the President also warned that lack of any progress during the May talks in Geneva could force the United States to step up new weapons development.

"Obviously, if we feel at the conclusion of [the May] discussions that the Soviets are not acting in good faith with us and that an agreement is unlikely, then I would be forced to consider a much more deep commitment to the development and deployment of additional weapons," he said.

"But I would like to forgo that decision until I am convinced the Soviets are not acting in good faith," he added.

In response to another question, Carter said he had "no evidence" that the Moscow talks ended without an agreement because of Soviet dissatisfaction with his campaign for human rights. He called this "an incorrect linkage" of issues if such was the case, and stated flatly, "I will not modify my human rights statements."

The President emphasized that he had not expected a quick agreement in Moscow.

"We had no indications either in direct or indirect communications with [Soviet leader Leonid I. Brezhnev] that they were ready to accept our positions."

He speculated that the Soviets rejected the more ambitious American plan because it is "so substantive and such a radical departure . . . that the Soviets simply need more time to consider it."

During the Moscow negotiations, the Soviet position did not waver from what it has been in the past - an insistence that limits be placed on the American cruise missile as a strategic weapon. The Soviets have maintained that the Backfire bomber is a medium-range aircraft.

Carter's optimism appeared to be shared by House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) Leaving the White House, O'Neill said, "Everything is going as anticipated."

The President used the term "hang tough" during a meeting with O'Neill and other congressional leaders whom he summoned to the White House yesterday afternoon after the Moscow talks concluded.

Asked by reporters to explain what he meant by the term which was attributed to him by Senate Minority Leader Howard H. Baker Jr. (R-Tenn.), Carter said:

"I think it is important for us to take advantage of an opportunity this year to negotiate not just a superficial ratification of rules by which we can continue the arms race, but to have a freeze on deployment and development of missiles and an actual reduction in launchers . . ."

"On those terms I intend to remain very strong in my position," he continued. "I don't think it is to our nation's advantage to put forward in piecemeal fashion additional proposals. Our experience in the past has been that the Soviet Union extracts from those comprehensive proposals those items that are favourable to them and wants to continue to negotiate the other parts of the proposal . . ."

In discussing earlier negotiations by the Nixon and Ford administrations, Carter said the Soviets do not contend that any secret accords were reached on the cruise missile.

Rather, he said, the stumbling block in ratifying the Vladivostok agreement centers on different interpretations of the agree said, the stumbling block in ratifying the Viadivostok agreement centers on different interpretations of the agreement's language, which the Soviets say would prevent deployment of the cruise missile and the Americans say does not affect the weapon.