President Carter urged the Soviet Union yesterday to join in an early accord on the stalled nuclear weapons pact, to ban mobile atomic missiles, and to give advance notice of intercontinental missile test firings.

The President's major departure in his first press conference was to bring into open discussion portions of traditionally secretive nuclear diplomacy while they are under negotiation. No other President, or Secretary of State, has done so to any comparable degree.

This departure was more striking than some of the nuclear proposals, which he described as "symbolic in nature." The most important of them was rejected by the Soviet Union when it was made by the Ford administration: to complete the 1974 strategic arms limitation talks (SALT) accord by setting aside temporarily the dispute over counting Soviet bombers known as Backfire, for American long-range cruise missiles.

Carter used the occasion to put a stamp of firm leadership on the controversy about his administration's nuclear intentions. He was determined, he said, "to demonstrate to the world that we are sincere" and curbing nuclear arms.

The President ranged confidently over a complex of issues that included his concern about the sale of concusion bombs to Israel, and his determination "to speak out strong and forcefully when human rights are threatened.

"This is not intended as a public relations attack on the Soviet Union," he said. The President said he has told the Soviet Union that unlike the previous Nixon-Ford administrations, he pursues no "linkage" strategy which requires avoiding the discussion of human rights, for fear of endergering nuclear negotiations.

As the President spoke to the nation television, Paul C. Warnke, Carter's controversial nominee to head the U.S. Arms Control and Disarmament Agency, was beginning to testify in his Senate confirmation hearings. Warkne's nomination is swept up in dispute about whether the Soviet Union threatens to gain nuclear strategic superiority over the United States.

Carter initially said, "At the present time, my judgement is that we have superior nuclear capability," and he then said, "I think that we are roughly equivalent, even though I think we are superior . . . "

In a nuclear war, the President said, either nation "could destroy a major part of the other nation if a major attack was made with losses in the neighborhood of 50 to 100 million if a large exchange was initiated."

It is "this kind of holocaust," he said, that makes it crucial to achieve "drastic reduction in dependence on nuclear weapons."

He said he has "complete confidence" in Warnke, and added that Warnke twice declined the post before agreeing to take it at Carter's insistence. Although critics have charged Warnke with advocating risky "unilateral" arms restraing, Carter said, "I obviously believe that we both have to take initiatives both the Soviet Union and the United States.

"Our decisions with the Soviet Union will be made public," the President said. Any agreement will obviously require senatorial approval, he added and "if I or Mr. Warnke or some other person . . . should make a mistake inadvertently, that mistake would be closely scrutinized by the public. . . "

The President, a former Navy nuclear submarine specialist, eagerly reached for opportunity to explain his thoughts on nuclear issues, which dominated the news conference. In the process, he discussed portions of his Feb. 1 talk with Soviet Ambassador Anatoliy F. Dobrynin, and his meeting yesterday with Huang Chen, chief of the liason mission of the People's Republic of China.

In his desire to conclude "a quick agreement" with the Soviet Union on the stalled SALT talks, Carter proposed, as he previously has indicated publicly, completing the basic accord reached by President Ford and Soviet leader Leonid I. Brezhnev at Vladivostok in November, 1974.

That intended pact set a limit of 2,400 on each nation's intercontinental missiles and bombers, of which 1,321 weapons on a side could carry multiple nuclear warheads.

The pact has foundered for 2 1/2 years primarily over a two-headed subsequent dispute: whether those ceilings should include, as the United States insists, what the Soviet Union calls a medium bomber, the Backfire, which the United States says has range enough to reach this country, secondly whether the limits apply to long-range cruise missiles, as the Soviet Union demands.

The United States has the technological lead in developing air, sea and land-launched cruise missiles, which akin to a jet aircraft, and which the Pentagon insists should be excluded from the Vladivostok accord.

The President said he is prepared to conclude swiftly what is known as a SALT II agreement that would omit the Backfire and cruise missile."And then in the Salt III talks, if necessary, put those two items back in for future discussion."

Although the Ford administration unsuccessfully tried the same approach, Carter administration sources say they believe it is worth another try. The Soviet concern has been that cruise missiles would be left unrestricted. The President sidestepped a question on whether he would favor restraining development of cruise missiles to alleviate Soviet concern.

In addition to the Backfire-cruise missile dispute, the President raised another issue of major current U.S. concern by suggesting a ban on deployment of long-range mobile missiles. This was unresolved in the 1972 SALT I accord.

Carter said the Soviet Union has begun to deploy what are known in the West as SS-20 medium-range missiles, which can be moved around in "a concealed way . . . "

The SS-20 "is very difficult to distinguish from the [Soviet] intercontinental missile called the SS-16," he noted. The SS-20 is two sections of the longer-range SS-16, and the U.S. strategists say the conversion can be made quickly, undermining the Vladivostok ceiling on intercontinental weapons.

If the Soviet Union would halt this deployment, Carter said, "It would mean we would not then perhaps spend the large amounts of money to develop a mobile system."

As another step forward, Carter noted that American intercontinental test missiles are launched from Vandenberg Air Force Base in California. The Soviet Union however, he said launches its test missiles "from the standard [military] silos."

Carter said a Soviet notice of "24 hours or 48 hours "of its test firings "would help a great deal" to ease tension about what was being fired.

The President also suggested a variation on his call for a total ban on all nuclear testing, to accommodate Soviet insistence on retaining the right to have "peaceful nuclear explosions," as permitted under pending, interlocking, U.S. - Soviet treaties.

Carter said that with a total test ban, the Soviet desire to use nuclear explosives "to divert the course of a river in the northern Russia" could be arranged as "provisio in the agreement . . . "

The President said that in his 90-day minute meeting yesterday with the Chinese ambassador, Huang Chen told him "the goal of the Chinese government was to reduce dependence on nuclear weapons to zero." This has been China's known position, that it will ban nuclear weapons if every other nation does so.