UNITED NATIONS, N.Y.--President Reagan told the U.N. General Assembly yesterday that "the ice of the negotiating stalemate could break" during the current round of Geneva arms talks with the Soviet Union.

Although Reagan said "a pall has been cast" over U.S.-Soviet relations by the Nicholas S. Daniloff '56 affair, he did not suggest the case would stand in the way of progress toward reducing both medium and long-range nuclear weapons.

Referring to the FBI's arrest in August of a Soviet employee of the United Nations accused of spying for the Kremlin, Reagan said: "Misusing the United Nations for purposes of espionage does a grave disservice to this organization. The world expects better."

Reagan's speech used stern words to criticize the Soviets, not only for their treatment of Daniloff, an American journalist arrested in Moscow, but also for their treatment of civilians in Soviet-occupied Afghanistan and their insistence on supporting Marxist-Leninist insurrections around the world.

Soviet Foreign Minister Eduard A. Shevardnadze listened to Reagan's address, making notes without signaling any emotion, and sat quietly as representatives of other nations applauded as Reagan finished speaking.

An Administration official, briefing reporters about the speech on condition he not be identified, said that after Reagan received a letter Friday from Soviet leader Mikhail S. Gorbachev, he asked that the speech be reviewed to "make sure the tone was not nasty."

But Reagan cited Daniloff's arrest and his subsequent confinement to Moscow on spy charges as "a particularly disturbing example of Soviet transgressions against human rights. The Soviet Union bears the responsibility for the consequences of its action."

The President gave no clue as to what those consequences might be.

Later, as he was going into a meeting here with foreign ministers of U.S. allies, Reagan told reporters: "I'm not going to comment now. Everything's too delicate."

On arms control, however, Reagan's tone was unusually conciliatory.

He said the Soviets, while unwilling to accept U.S. proposals for a 50 percent cut in intercontinental missiles, bombers and submarines, have "now embraced our idea of radical reductions in offensive systems."

The Soviets had rejected the initial U.S. demand because it would have forced abandonment of many of the heavy land-based missiles that are the core of their nuclear arsenal.

Although their most recent offer of less drastic cuts wasn't acceptable, Reagan said, it appeared to be "a serious effort." Because of this, he said, the United States is "prepared to consider" more modest reductions as an interim step toward the eventual elimination of offensive nuclear weapons.

"So, there has been movement," Reagan reported. He added that the United States still seeks "total elimination" of intermediate-range missiles in Europe--his so-called zero option.

"The ice of the negotiating stalemate could break--if both sides intensify their efforts in the new round of Geneva talks" that opened last week, Reagan told the world leaders gathered for the opening of the annual General Assembly session.

Reagan, however, gave no indication of significant U.S. concessions on his Strategic Defense Initiative, "Star Wars." The Soviets claim that building such a space-based shield against nuclear missiles only forces them to build more offensive weapons to make sure they could penetrate U.S. defenses in the event of war.

Reagan said he was prepared to talk to the Soviets about the "ground rules" for shifting toward strategic defense systems, and confirmed that he offered Gorbachev a plan for a sharing of research on Star Wars.

Although the President's offer, disclosed by sources after he made it in a letter last July, was his first formal offer to negotiate on Star Wars, it would not prevent unilateral deployment of a defensive system.