VENICE, JUNE 5 -- President Reagan today sought to reassure Western Europe that the American commitment to Europe's military defense would not be diminished by a U.S.-Soviet accord to slash their arsenals of European-based nuclear missiles.

In a speech televised throughout Western Europe, the president said that NATO nations "must improve our conventional defense capabilities, difficult and expensive as that might be" in order to counter the strength of Soviet conventional military forces.

"The United States will not waver in our commitment to the defense of Europe," Reagan said. "We will sustain the credibility of NATO's doctrine of flexible response, which has served us well and remains the center of alliance strategy."

A senior official said Reagan was mindful of European concerns that a pending accord to eliminate medium-range missiles from Europe would represent a weakening of the American defense commitment to the Continent.

He said the president would respond to these anxieties throughout his nine-day trip to Europe. Asked what Reagan's priorities are, the official replied, "Arms control, arms control, arms control."

The accord would remove from Europe all medium-range missiles carrying nuclear warheads with ranges of between 600 and 3,500 miles. It would also get rid of superpower missiles in the 300-to-600 mile range.

The West German government endorsed the outline of this proposal Thursday but said the accord must not remove the U.S.-controlled nuclear warheads on 72 West German Pershing IA missiles, which can hit targets up to 450 miles away.

Both the West German government and the Reagan administration regard these missiles as third-country systems, like those of the British and French, that are not covered by the proposed treaty. The Soviet Union has taken the position that the West Germans can keep the missiles but the U.S.-controlled warheads must be removed.

The senior official, who discussed Reagan's priorities on condition he not be identified, acknowledged that this difference had the potential to be "a deal-breaker." But he expressed hope that West German public opinion would ultimately favor the U.S.-Soviet agreement even if this meant that the government of Chancellor Helmut Kohl has to give ground on the issue of the Pershings.

In his speech today, Reagan conceded that "some hard questions remain" before the accord can be reached but said "the prospects are good." He coupled his optimism about arms control with a denunciation of Soviet policies in Europe since the end of World War II, beginning with the observation that Soviet dictator Joseph Stalin had refused to allow East European nations to participate in the Marshall Plan, which rebuilt Western Europe.

"We've heard a lot about the Soviet desire to participate in the world economy -- to no longer be the odd man out," Reagan said. "Well, the ground rules remain the same as they were 40 years ago. No playing the spoiler. No manipulation of world organizations for political gain. Open your economy. Open your political system. Open your borders. Let your people go."

Referring to Soviet leader Mikhail Gorbachev's policy of glasnost, or openness, without mentioning him by name, Reagan said, "We hope that the first few tokens of change in the Soviet Union signal a real desire to open up that closed society."

Reagan also said the treaty to eliminate medium-range missiles "will not be the end but the beginning of the arms reduction effort. Our top priority remains deep, equitable and verifiable reductions in intercontinental nuclear arms."

In his speech today, transmitted by the U.S. Information Agency to Western European countries, Reagan said agricultural subsidies will be a major topic of discussion among the seven industrialized democracies participating in the 13th annual economic summit that opens here Monday.

Reagan called for "setting a goal of a subsidy-free world" for agricultural commodities by the year 2000 despite the increase of U.S. agricultural subsidies during his presidency.

He also denounced protectionism, comparing it to the "evil of drugs" because "it will end up destroying all those who use it."