Reagan writes to Mr. Tsvetkov; 'USSR must ease arms stance'

The main thrust of Soviet negotiating tactics remains an effort to disrupt the deployment of American missiles in Western Europe, administration officials say.

Officials say that the Soviets have shown some flexibility in the START talks at Geneva aimed at reducing strategic nuclear missiles. But officials add that this flexibility has not applied to the central issues. They conclude that the Soviets are dragging their feet in these and in parallel talks on intermediate-range missiles in the hope that frustrated West Europeans will bring more pressure to bear on the United States.

The ultimate Soviet hope, the officials say, is that the West Europeans will call for a delay in the deployment of new American medium-range missiles, the first of which are now scheduled to be placed in West Europe in December.

In an effort to show American goodwill, meanwhile, President Reagan has reaffirmed that he has ''no higher goal, no more urgent task, than to reduce the threat of nuclear war to all the world's people.''

But the President accused the leaders of the Soviet Union of not having demonstrated enough willingness to compromise to reach a genuine arms control agreement. The President's pledge to work ''vigorously'' toward nuclear arms control came in a letter to a Bulgarian citizen who had written to him expressing concern that the American leader might be preparing for a nuclear war.

In Moscow, Soviet President Yuri Andropov told visiting US senators on Thursday that the Soviets would impose a unilateral moratorium on the launching of anti-satellite weapons into outer space as long as the US and other nations did so as well.

The problem with this proposal, administration officials say, is that the Soviets have moved ahead in the development of anti-satellite weapons. As Alan Romberg, the State Department spokesman, put it, ''The Soviets have the world's only operational anti-satellite systems,'' which they have continued to test for more than a dozen years.

In the 1960s, both the US and Soviet Union fielded anti-satellite, or ASAT systems. According to a new study commissioned by the Union of Concerned Scientists, the US eventually dismantled its ASAT systems and adopted the position that its national security would best be served by abstaining from competition in ASAT weaponry. The Soviets, on the other hand, continued sporadic testing of what the study describes as ''a rather primitive device that can attack objects in low earth orbits.''

The US ASAT system consists of an interceptor that is carried into space on a two-stage rocket. The rocket is launched from an F-15 fighter flying at high altitudes. The first tests against space targets were scheduled for late 1983 or early 1984.

In addition to offering the moratorium on ASAT weapons, Mr. Andropov has assumed a more threatening tone when it comes to the scheduled deployment of new American medium-range missiles in Western Europe. Andropov indicated to a visiting US union leader, William Winpisinger, on Wednesday that if the deployment went ahead, the Soviets would regard the arms control talks in Geneva as effectively over and would concentrate on taking ''defense countermeasures.''

State Department spokesman Romberg responded on Thursday that Andropov's comments had to be viewed in light of the Soviets' ''intensifying public campaign to undermine support'' in Europe for the US medium-range missile deployment.

''We are negotiating seriously in Geneva and believe that satisfactory results can be achieved if the Soviet Union decides to negotiate responsibly and makes serious proposals on the central issues . . . ,'' Romberg said.

Romberg said the medium-range American missiles were being deployed to counter an already massive Soviet deployment of 351 SS-20 missiles carrying a total of 1,053 warheads.

Reagan's renewed pledge to negotiate seriously came in a letter, dated Aug. 3 , sent to a 90-year-old Bulgarian citizen, D. Tsvetkov, who had written a letter to the President expressing concern that ''something terrible is coming for the future of mankind.'' Copies of both letters were obtained by this newspaper from a US official.

''My deep conviction is that you are preparing for . . . a deadly nuclear war ,'' the Bulgarian wrote to Reagan.

Reagan replied that the US is now embarked on ''one of the most ambitious and comprehensive programs for arms control ever.'' The President mentioned the two sets of talks in Geneva, his proposals to eliminate chemical weapons, to reduce the risk of war by accident or miscalculation, and to make substantial, verifiable reductions in conventional forces in Europe.

''We will continue to pursue arms control negotiations vigorously, for I am determined to strive for equitable and verifiable agreements that will provide for substantial reductions and greatly enhanced security,'' said Reagan.

The President accused the Soviets of engaging in ''an unprecedented military buildup.''

''Although Soviet leaders have always proclaimed their interest in arms control and disarmament, their actual behavior has been quite different throughout the 1970s and '80s,'' Reagan said.