The Soviet Union marked the opening of Soviet-U.S. strategic arms talks in Geneva today with expressions of doubt about American sincerity and charges that the Reagan administration is "pressing forward with material preparations for war."

Defense Minister Dmitri Ustinov charged that the United States is responsible for a "sharp deterioration" of the international situation and that the U.S. aims to "attain military superiority over the Soviet Union."

Having set that aim, Ustinov was quoted as telling the graduates of Soviet military academies today, "The United States and NATO are intensively building up armaments, especially nuclear armaments, modernizing their armies and pressing forward with material preparations for war."

The government news agency Tass quoted Ustinov as saying that the Soviet defenses would be maintained "at a proper level," that Soviet weapons systems are being "steadily perfected" and that "the level of combat efficiency" is being increased.

Political observers noted that the publication of Ustinov's remarks was in contrast to past practices, when the Soviets sought to create what they call "favorable atmospheres" at the start of major negotiations.

Well-informed Soviet sources said privately that Moscow remains deeply suspicious about American intentions at Geneva. Moreover, the recent resignation of Secretary of State Alexander M. Haig Jr. has raised fresh questions here about the course of U.S. foreign policy, according to the sources.

Other expressions of Soviet doubts came in a Tass commentary, which described the chief American negotiator, Edward Rowny, as "a rabid opponent" of the SALT II treaty, and a full-page article by Leonid Zamyatin, the chief spokesman for the Soviet Communist Party's Central Committee, which accused Washington of intensifying its ideological war against the Soviet Union.

"U.S. propaganda, like U.S. politics, has no positive goals or ideals," Zamyatin wrote in the newspaper Literary Gazette. "It is destructive, permeated with the cult of force and the spirit of militarism."

In the only direct commentary on the Geneva talks, Tass raised the question of U.S. intentions, suggesting that the Reagan administration may have decided to enter the talks in an effort to deflect antinuclear movements in Europe and the United States.

That decision, Tass said, is "only a small part" of the task before the two countries.

"Talks can be conducted in the course of a year, three, five and even 10 years but with no agreement ever reached. Consequently, only an insignificant part of the way that is to be covered has been traversed" by the two sides, Tass said.

Tass underscored Soviet reservations about President Reagan's proposal to cut the numbers of land-based nuclear weapons, in which Moscow enjoys numerical superiority. It said Reagan deliberately glossed over the issue of sea and air-based nuclear arms "where superiority, and large superiority for that matter, is on the U.S. side."

The commentary restated Soviet readiness to freeze its strategic weapons quantitatively and "limit their modernization to a maximum." It said that the Geneva talks would have to be conducted on the basis of equal security for both sides, that their goal should be to "limit and reduce" the number of weapons "rather than serve as a cover for the continuing arms race and breaking the present parity" and that they should "preserve everything positive" achieved in the previous SALT negotiations.

"It must be absolutely clear to the United States, its NATO allies and all countries of the world that the unshakable basis for the talks between the United States and the Soviet Union, and for an agreement between them, is equality and equal security," the Tass commentary said.

The principle is seen as essential for both the strategic talks, which started yesterday, and the negotiations to reduce the number of medium-range nuclear weapons in Europe, which got under way earlier this year.