The new Soviet leadership appears to be signaling that it is on the threshold of a decision to escalate the arms race, although Defense Minister Dmitri Ustinov's message last night also seemed to indicate a preference for negotiating arms-limitation agreements with the United States.

But Ustinov stressed that the Soviets did not want such agreements at any price and that they were prepared to meet the Reagan administration's challenge of a new round in the arms race if necessary.

Well-informed observers here interpreted Ustinov's remarks, overall, as a shift back to a hard line that was emerging in the last weeks of Leonid Brezhnev's life, and particularly in his Oct. 27 meeting with the armed forces chiefs. At that time, Brezhnev forecast a new wave of Soviet weapons modernization to counter what he called an "unprecedented" U.S. arms buildup.

While the prevailing view among foreign and Soviet observers is that a new Soviet move is in the offing, some Western diplomats here maintained that Ustinov's interview with the news agency Tass was a "propaganda ploy" to influence the outcome of a congressional debate in Washington on President Reagan's plan to deploy 100 of the controversial MX missiles in a Dense Pack formation in Wyoming.

The timing of Ustinov's interview and its relatively moderate and reasonable tone generally are believed to represent Moscow's last-ditch effort to derail the MX system. But Ustinov's warning that the Soviets would match the MX with a similar weapon closely followed the disclosure that the Soviet Union had tested a new intercontinental ballistic missile within the past three weeks.

The Soviet missile is believed to be the SS16, a three-stage version of the highly accurate, mobile SS20 missile. That two-stage missile has upset the military balance in medium-range weapons in Europe.

Since the MX would substantially increase the perceived vulnerability of Moscow's huge land-based SS18 and SS19 missiles, military specialists here believe that Moscow has a powerful incentive to deploy a new and mobile intercontinental ballistic missile. In this view, the current confrontation between the two superpowers may produce a potentially fatal setback to arms control for the next decade or more.

The reason, according to this view, is that a mobile and accurate Soviet intercontinental weapon would make future arms-control talks far more complex than anything encountered in the past. Given the large Soviet land mass, such missiles could be moved around or hidden in a way to make for insurmountable verification difficulties.

Unlike the United States with its constraints created by environmentalists, concerned citizens and anti-nuclear activists, the Soviet Union can place a new rocket such as the SS16 on wheels without much concern about public reaction.

Experts here say the main reason the Soviets have refrained from developing the SS16 fully is that its deployment would drastically reduce chances for future arms-control agreements with the United States.

Ustinov went at great length to show that the Soviet Union did not enjoy military superiority over the United States. He said the MX program and the scheduled deployment of new medium-range U.S. nuclear rockets in Western Europe were designed to change the existing "strategic parity" in America's favor.

He firmly asserted that if challenged, the Soviet Union would deploy a new intercontinental ballistic missile "in the same class" as the MX. He said that Moscow would "effectively repond" to other threats from Washington, apparently an allusion to the deployment of U.S. medium-range missiles in Europe.

Brezhnev's death in November and the meeting between his successor, Yuri Andropov, and Vice President Bush and Secretary of State George P. Shultz raised the prospect that the two superpowers might make a fresh start in tackling their differences.

In his hard-hitting but thoughtful speech on Nov. 22, Andropov talked about Soviet readiness to normalize U.S. relations "on the basis of reciprocity and equality." Mankind, he said, "cannot put up with the arms race forever unless it wants to put its own future at stake."

But, he said, no one should expect the Soviet Union to disarm unilaterally -- an indirect reference to Reagan's "zero option" plan that calls for dismantling of all Soviet medium-range nuclear rockets in the European theater if the United States were to abandon the planned deployment of its new medium-range missiles in Europe. "We are not naive people," Andropov said.

In less than 24 hours, Reagan announced his MX deployment plan. That move is thought to have angered the Soviets and raised new fears about Reagan's intentions.

Ustinov's interview last night appears to reflect a return to the hard-line Soviet position that emerged during the October reassessment of Soviet security needs. That reassessment is believed to have been prompted by behind-the-scenes criticism by the military establishment of the Kremlin's response to Washington's arms buildup.

The prevalent view expressed by Soviet experts is that there is little to expect from relations with the United States as long as Reagan remains in the White House.

That Ustinov clearly left the door open to U.S. overtures was seen by foreign observers as a deliberate gesture toward Western Europe to demonstrate Moscow's continued interest in arms control. According to this view, Western Europe will increasingly become the focus of Soviet attention as there are hopes here that West Europeans may evenutually block at least a major part of Reagan's plan -- the deployment of the medium-range Pershing II and cruise missiles on the continent.