Georgy Arbatov, the senior Kremlin adviser on U.S. affairs, has expressed concern about a "tremendous deterioration" in Soviet-American relations, saying that "the situation is worse now than at any time since the Cuban missile crisis."

"We have entered a very dangerous period," Arbatov said in an interview yesterday. "The door is open" for talks, he said, "but everything looks rather sinister. In this atmosphere, which is becoming denser and denser, any spark could lead to a crisis."

He said the Reagan administration's political and military policies have brought about a threat to international stability, adding, "Its each step is more destabilizing than the previous one."

Arbatov spoke against the backdrop of sharp warnings to the West earlier this week by Marshal Dmitri Ustinov, the defense minister, and Marshal Viktor Kulikov, the Warsaw Pact commander. The comments indicate that the Soviet high command expects that the United States will proceed with the deployment of 572 cruise and Pershing II missiles in Western Europe. The missiles are due to be deployed beginning in December.

Ustinov, speaking in East Germany Wednesday, threatened nuclear devastation of both the United States and Western Europe if these weapons are used against the Soviet Union. Kulikov in an article published the same day talked about growing "war danger" and Soviet readiness to counter Reagan's arms challenge.

Moscow's new, harder tone toward the Reagan administration was set at Foreign Minister Andrei Gromyko's press conference a week ago. Gromyko, who had not appeared before foreign correspondents here in four years, firmly rejected Reagan's offer to negotiate a revised arms agreement in which the United States would reduce the number of missiles to be deployed in Europe if the Soviets agreed to scrap some of their missiles already deployed.

Although Gromyko was cautious, the thrust of his remarks reflected the conviction that the Geneva negotiations on arms control were not likely to make any headway for the rest of the year and that Washington is determined to deploy new weapons at all costs.

Arbatov, who has close ties to Soviet leader Yuri Andropov, said the deployment of the 572 U.S. missiles would require a dual Soviet response.

"First, we will have to do something in the European theater to restore the balance" if the deployment takes place, Arbatov said. "And second, we would have to do the same in the strategic field because these weapons have a dual mission." From Moscow's point of view, the introduction of the new missiles changes not only the balance in Europe but also the rough strategic parity between the two superpowers.

Professing lack of knowledge of military details, Arbatov would not discuss the threat by the Soviet leader to place the United States in an "analogous" position should the new medium-range missiles be deployed. Arbatov referred to this several weeks ago when he wrote in an article that Soviet weapons would have to be brought close to America's shores.

But another senior Soviet official, Vadim Zagladin, was quoted yesterday by the Hungarian news agency MTI as saying that Moscow would "never break" an understanding with the United States not to introduce Soviet missiles in Cuba. That pledge was part of a deal that ended the 1962 Cuban missile crisis.

However, Zagladin, a first deputy departmental head of the Communist Party's Central Committee, also was quoted as saying in response to the U.S. plans for the missile deployment that "if only one extra missile is deployed, we shall be compelled to react. First of all, we shall have to balance the threat of a new danger, and it is an extremely serious danger.

"The American missiles can reach Hungary in three minutes and the Soviet Union in five. We need a defense system which can reach the United States in three or five minutes. How? By technical means--and not from Cuba."

Arbatov confirmed reports that the Soviet Union has tested successfully a cruise missile of its own recently. But the deployment of long-range cruise missiles, he said, would destroy the arms control process because of the problem of verification.

It would be a "heavier blow to arms control and stability than the introduction of MIRVed weapons,"--weapons with multiple independently targeted warheads, he said. The United States was the first to introduce these weapons and enjoyed a significant advantage in the early 1970s. At that time, Washington refused to discuss curbs on MIRVed systems. Once the Soviets developed it, they gained a significant advantage because their rockets were more powerful and could carry more of the warheads.

"If you deploy long-range cruise missiles, what can we discuss?" Arbatov asked. He criticized Reagan's "Star Wars" speech, in which the president proposed U.S. research and development of laser, particle beam and other exotic weapons. Arbatov said this was "a heavy blow to stability even though these weapons do not exist," suggesting that Soviet scientists may begin to explore various new arms ideas simply as a reaction to Reagan's speech.

The question, he said, "is one of enlightened self-interest and not whether Americans or Russians are good guys or bad guys."

"The danger of confrontation and conflict has become much more real now," he continued. At the time of the Cuban missile crisis, the Soviet Union and the United States had only a fraction of the weapons they possess now--"less than the French and British have now."

A nuclear alert during the 1973 Arab-Israeli war "was not dangerous" because political relations were far better.

Today, Arbatov continued, "I don't see any sense of responsibility" in Washington. "I don't understand how people can do things without calculating and foreseeing any of the consequences."

The Soviet Union, he said, will not be mesmerized by "some useless and exotic weapons" and will not copy them "just because the Americans are making them." He also asserted that Moscow would not shift its arms industry in a direction in which it is being pushed by Washington because this would mean the "annulment of our capital investments."

The Reagan administration, he said, "is trying to provoke the Soviet Union into doing something harmful to the Soviet economy, to negate our capital investments and to make us spend so much that we would bleed white."

"We will have to spend" more on new weapons, he added, and "we already may be doing something. I don't have any particular information but logically it should be so."

Arbatov sought to dispel an impression that Moscow finds itself isolated in Western Europe and faced with more hostile governments than only a year ago. He called this "a temporary episode" and said there would be "new developments" there in the coming months.

He indirectly talked about the recent expulsions of Soviet officials from several West European countries. He said they were part of a campaign "orchestrated by the United States to discredit" Moscow and its foreign policy. But these "hysterical things will be short-lived."

The Reagan administration is "trying to raise the temperature even higher," he added. The Soviets do not intend to "contribute" to that effort.