The United States, in the latest installment of a propaganda war over curbing nuclear arms, yesterday denounced the Soviet Union's newly proclaimed negotiating position on medium-range missiles in Europe.

A State Department statement read by spokesman Dean Fischer rebutted the substance of the proposal made public in Moscow Tuesday. Fischer also said it is "unfortunate that the Soviet Union apparently intends to conduct these negotiations through their press, rather than at the conference table."

The public exchange of the past 24 hours gave no indication of substantial progress in the 10 weeks of negotiations between the two sides in Geneva on the limitation of European-based nuclear arms.

The absence of initial movement on either side is not surprising or particularly disturbing, in the view of veteran observers of such negotiations. However, the increasingly open propaganda battle surrounding the negotiations is unusual and appears to diminish the chances that the talks ultimately will succeed.

The Soviet proposal announced Tuesday by the government news agency Tass calls for phased reductions to "300 units" of medium-range weapons on each side by 1991, a reduction of more than two-thirds from Moscow's calculation of about 1,000 "units" on each side now.

Fischer declared that "by selective inclusion and exclusion of systems, the Soviets base their proposal on a spurious assessment of the existing balance."

He said that under its proposal, the Soviet Union would be able to continue modernization of medium-range weapons, including new deployment of SS20 missiles, while forcing the United States to cancel its planned deployment of modern Pershing II missiles.

U.S. intelligence calculates that the Soviets have substantially fewer than 200 of their most modern SS20 medium-range missiles based west of the Urals. In this view, therefore, a limit of 300 "units" by 1991 would permit the Russians an increase.

The U.S. position in the Geneva talks is that the Soviets have a large advantage in land-based intermediate-range missiles. If the Russians will eliminate these missiles in place, the United States offers to cancel the deployment of Pershing II missiles designed to catch up.

The Soviet calculation, on the other hand, is that two sides have roughly an equal number of weapons. This takes into account American warplanes capable of delivering nuclear weapons as well as American, British and French missiles based on submarines and other ships at sea.

The responsibility for the war of words over the negotiating positions is difficult to assess.

President Reagan used a nationally televised speech Nov. 18 to launch the Nov. 30 Geneva negotiations. Reagan's speech and background briefings set out the thrust of the U.S. position.

After two months of mostly confidential talks, Soviet President Leonid Brezhnev, in a Moscow speech Feb. 3, charged the United States with dragging its feet, saying its proposal was not serious, and publicly announced a Soviet proposal to reduce medium-range arsenals in Europe by two-thirds.

Reagan, in a White House statement Feb. 4, announced that the United States had submitted a draft treaty to the Geneva negotiations Feb. 2. The White House called its proposal "serious and far-reaching" and rejected the accusation that it is stalling.

The Tass statement Tuesday added detail to the proposal Brezhnev had mentioned earlier. The proposal apparently was submitted to the Geneva talks weeks ago.