An American delegation has bluntly warned the Soviet Union that the SALT II agreement cannot be ratified without major changes and said that Soviet officials told them Moscow will consider new arms proposals that could rescue the treaty.

In the first face-to-face bilateral exchanges here since the American presidential election, the private U.S. group, told senior Soviet foreign policy and arms advisers they must accept "as fact that SALT II will not be ratified" unless it is changed in major ways, representatives of the group said.

Lt. Gen. Brent Scowcroft, a former Ford administration national security adviser who has been mentioned as a candidate for that post under President-elect Reagan, told a press conference at the end of the three days of talks today that the Soviets "do not rully accept this position." Nevertheless, delegation leader William Scranton said, the Soviets, "while stating that the agreement is signed and they'll stand pat, also said it is the United States' duty to propose any desired changes." p

Scanton, speaking for the nine-member delegation from the U.S. United Nations Association, said the Soviets sought, and received, some informal proposals for treaty changes, but he refused to discuss them.

Soviet President Leonid Brezhnev has said the SALT II agreement he signed with President Carter at the June 1979 Vienna summit must be approved by the U.S. Senate without changes and that attempts to reopen negotiations would seriously jeopardize detente. But last week, new Soviet Premier Nikolai Tikhonov said the Soviet Union is ready for any constructive arms limitation proposals.

While the Soviets were described as not pleased with what they heard from the Americans this week, Scranton said they did not criticize Reagan and assured the delegation that any new effort to open a dialogue would receive a positive response.

With both sides agreeing grave potential dangers face them, he said, the Soviets asserted the two leaderships have a unique "opportunity to consider Soviet-American relations anew," because the Kremlin also is reassessing its policies in preparation for its Communist Party congress next February.

The Americans said the Soviet advisers were told they can expect tough, but "businesslike" dealings from the new administration on behalf of a nation clearly aroused and worried by Soviet military intervention in Afghanistan and concerned about a possible future Soviet military move in Poland.

Although the Americans were a private group, the presence of Scowcroft gave special meaning to the talks that reportedly was not lost on the Soviets. gDr. Georgi Arbatov, head of the U.S.A. Institute and a senior American affairs adviser to Brezhnev, led the Soviet side.

The Americans said the Soviets were eager to get a reading on the new administration's defense spending plans.

"Unfortunately for them," one source said, "their five-year plan must be adoped well before they can know what the new administration is going to do." But the U.S. group was said to have told the Soviets, who have their own economic problems after two straight harvest disasters and the need to bolster strike-plagued Poland with food and money that they can expect steady increases in arms outlays by Reagan.

The Americans emphasized the new administration's intention to link bilateral issues in foreign policy, but the Soviets are said to have complained that linkage is only a ploy to deny them parity as a world power with the United States.

The talks started Wednesday, with the Americans denouncing the Afghan invasion as a major blow to bilateral relations, and warning that any military move in Poland, which has just asked Washington for a $3 billion loan, would destroy any possibility of restoring relations for many years to come.

The Soviets are said to have complained that the United States has "no vital interests" in Afghanistan, and to have rejected the group's position that the intervention poses unavoidable threat to the West by bringing the Soviets closer to vital Persian Gulf oilfields, according to U.S. participants.

It was understood that on the SALT issue, the Americans argued that the agreement should deal with so-called "heavy" missiles as well as diferential limits on multiple warhead missiles and the Soviet Backfire strategic bomber. lThe pending treaty, which Reagan has said he will not support, has come under conservative attack for failing to safeguard the United States in this part of the strategic equation.

The Soviets were said to have complained bitterly that the U.S. position amounts to requiring Moscow to live by the terms of a treaty which the United States itself rejects until a new agreement can be concluded. The Russians said new talks could take years, as the last negotiation did.

One U.S. source described the Soviets as "smug" over Moscow's belief that the Atlantic alliance is deeply troubled while Soviet relations with Western Europe are improved. The Americans said they emphasized to the Soviets that disagreements over defense issues among the allies should not be mistaken as fundamental trouble for Washington in its overall relations with the Western countries.