President Reagan assured Europeans yesterday that a pending U.S.-Soviet treaty to eliminate medium- and shorter-range nuclear missiles would not diminish the U.S. commitment to the security of Western Europe.

In a speech broadcast throughout Europe, the president strongly defended the merits of the intermediate-range nuclear forces (INF) treaty that he and Soviet leader Mikhail Gorbachev are scheduled to sign here Dec. 7.

Reagan also said he hopes that the accord would be "the first big step" to a treaty that would halve the strategic nuclear arsenals of the two superpowers.

Addressing European concerns that the treaty would put Western European nations at a military disadvantage against larger Warsaw Pact conventional forces, Reagan said, "The commitment of the United States to the alliance and to the security of Europe -- INF treaty or no INF treaty -- remains unshakable."

"Over 300,000 American servicemen with you on the continent and our steadfast nuclear guarantee underscore this pledge," Reagan said. "Those who worry that we will somehow drift apart or that deterrence has been weakened are mistaken on both counts."

The speech, taped Tuesday, was broadcast on the United States Information Agency's "Worldnet" and the Voice of America.

Reagan reiterated his commitment to his missile-defense plan, the Strategic Defense Initiative (SDI), saying that "we won't bargain away SDI, which offers the promise of a safer world in which both sides would rely more on defenses -- which threaten no one -- than on offensive forces."

Returning to a formulation he has used on several occasions since his summit with Gorbachev a year ago in Iceland, Reagan said that "what is totally unacceptable . . . is the Soviet tactic of holding these offensive reductions hostage to measures that would cripple our Strategic Defense Initiative."

The president took a softer line a week ago when he announced the summit at the White House in the presence of Soviet Foreign Minister Eduard Shevardnadze. He said then that he thought U.S. and Soviet negotiators had "made some progress at strategic defense" because the Soviets were no longer calling for elimination of SDI as "a flat demand."

In another passage of his speech designed to reassure Europeans, the president said "we have agreed with our allies that the existing imbalances in conventional forces and chemical weapons must be redressed prior to any further nuclear reductions in Europe."

However, Robert J. McCartney of The Washington Post Foreign Service reported from Bonn that West German Foreign Minister Hans-Dietrich Genscher appeared to contradict this statement in a radio interview. He said Genscher apparently was unaware of Reagan's speech.

Asked by Radio Bremen whether negotiations on conventional weapons or short-range nuclear missiles had 'priority' for the West German government, Genscher said, "We are equally affected by both . . . . We must dedicate ourselves to these negotiations with the same intensity."

He added that negotiations on chemical weapons must be pursued with equal intensity.

Genscher and other leaders of Chancellor Helmut Kohl's parliamentary group strongly reaffirmed keeping open the possibility of future U.S.-Soviet negotiations on reducing arsenals of nuclear missiles with a range of less than 300 miles, McCartney reported.

The statement came in response to reports from the NATO Nuclear Planning Group meeting in Monterey, Calif., that NATO was considering dropping a commitment made at West German insistence at a meeting in Reykjavik in June to include short-range weapons on the list of disarmament topics to be considered after an INF treaty.