Foreign Minister Andrei Gromyko declared yesterday that the Soviet Union would be ready immediately after the signing of a SALT II accord to begin a new round of talks aimed at achieving "a substantial reduction" in the nuclear arsenals of the superpowers.

U.S. officials said Gromyko's statement, made only hours before he was scheduled to fly to Washington for a crucial weekend of strategic arms limitation negotiations, marked the first time the Soviets have publicly agreed that the goal of SALT III should be deep cuts in strategic armaments such as those proposed last year by President Carter.

Gromyko's address to the U.N. General Assembly's special session on disarmament was notably mild in tone, and observers speculated that the Kremlin may be becoming a bit apprehensive over reports of growing congressional pressure on the Carter administration to postpone conclusion of a SALT II accord.

Gromyko, who will be discussing the remaining obstacles to a SALT II agreement in weekend talks with Secretary of State Crys Vance and President Carter, said he felt "possibilities exist for resolving the remaining issues."

"If we miss this chance, then in certain highly important areas we could reach a point beyond any possibility of concluding appropriate agreements will be altogether nonexistent," Gromkyo said. "Certain types of weapons which are being developed simply do not lend themselves to mutual control over their quantity or quantiative characteristics."

Gromyko added that "immediately after signing the agreement now being prepared, the Soviet Union would be ready to enter into negotiations which should lead - with all the necessary factors taken into account - to a substantial reduction, and I stress reduction, of the levels of strategic arms and to a further limitation of their qualitative improvement."

Many delegates, including several from the United States, felt Gromyko's address was relatively restrained in view of Vice President Mondale's sharp criticism of the Soviet Union two days earlier for escalating the arms race.

As expected, the Soviet foreign minister repeated the Kremlin's call for a ban on the neutron warhead that has been developed by the United States, terming it "a particularly vicious and cruel means of mass destruction, intended specially to annihilate all things living."

President Carter has temporarily postponed production of the warhead, but the United States has rejected the idea of a treaty banning just this one type of nuclear weapon.

Gromyko also responded to Mondale's attack on Soviet development of the mobile, medium-range missile known at the SS20, accusing the vice president of trying to "befuddle people" and whip up the arms race by "harping on the old stories like that of a Soviet military threat."

U.S. Ambassador Andrew Young said, "Gromyko's speech was not as tough as I expected." For the most part, Gromyko focused on the progress being made by the superpowers in various arms control forums, and he also spelled out in greater detail several recent Soviet proposals.

Gromyko appeared to attach the highest priority to the proposal Soviet President Leonid Brezhnev unveiled earlier this spring for a cessation of the production of all types of nuclear weapons.

Gromyko urged the special session to establish a preparatory committee to decide which nations - and how many - would take part in talks aimed at halting production of nuclear weapons. He asked the special session to set a date for the start of the negotiations.

U.S. officials later said they were perfectly willing to study this proposals, but added that the problem which trying to halt production of nuclear weapons is that there is no reliable way to verify that a country has actually done so.

"You can't monitor something like this with satellites or aerial reconnaissance," an official said. "You can almost fabricate a nuclear weapon - that is, put it together - in a garage, and outside of a few feet or yards, it does not emit anything you can pick up.

"We just don't see how you can halt production of nuclear weapons, and convey to anyone else any confidence that you have done so," he said.

Gromyko also updated an old Soviet proposal that the superpowers cut their military budgets by 10 percent, this time proposing that the United States, Soviet Union, Britain, France and China all agree to reduce their military budgets "in absolute figures."

This would hardly be possible, U.S. officials suggested, unless the Soviet Union was to become much more open and candid than it ever has been about its military spending.

Gromyko also proposed that the special session consider a treaty banning where there are so much weapons at present."

While U.S. officials appeared interested in this proposal, they suggested that it might limit Western defense options. Some members of NATO like Norway do not have nulcear weapons based in their countries at present, but might conceivably wish at some unforeseeable point in the future to reconsider, they suggested.

The sentiment of one NATO ally, however, appeared to be running in the opposite direction.

Canadian Prime Minister Pierre Elfott Trudeau told the disarmament assembly that his country had "withdrawn from any nuclear role by Canada's armed forces in Europe."

Moreover, Trudeau said Canada was "now in the process of replacing with conventionally armed aircraft the nuclear-capable planes still assigned to our forces in North America.

"We are thus not only the first country in the world with the capacity to produce nuclear weapons that chose not to do so," Trudeau said. "We are also the first nuclear-armed country to have chosen to divest itself of nuclear weapons."

West German Chancellor Helmut Schmidt also addressed the disarmament session yesterday.

Noting that his country had renounced the production of chemical weapons and submitted to international controls, he invited all countries interested to come to Germany to "see for themsleves" that a treaty banning chemical weapons is verifiable.

"We are convinced that it is possible to work out an international verification system, including on-site inspections, which would not prejudice the legitimate interests of industry and research," Schmidt declared.