UNITED NATIONS, N.Y., SEPT. 21 -- President Reagan called on Iran today to "clearly and unequivocally" accept a cease-fire in its seven-year-old Persian Gulf war with Iraq or face a global arms embargo led by the United States.

Reagan, addressing the opening of the U.N. General Assembly session, demanded that Iran's president, Ali Khamenei, who is scheduled to address the United Nations on Tuesday, "state whether . . . or not" Iran accepts the cease-fire resolution approved unanimously July 20 by the U.N. Security Council.

"If the answer is positive, it would be a welcome step and major breakthrough," Reagan told the world body. "If it is negative, the council has no choice but rapidly to adopt enforcement measures."

Just hours later, American military helicopters from the U.S. Persian Gulf naval force fired on an Iranian Navy vessel after it was reportedly sighted laying mines near gulf sea lanes plied by oil tankers. Reagan was told of the incident while he was returning to Washington aboard Air Force One.

Reagan in his speech urged the Soviet Union to join the effort to end the war, and called on Moscow to stop making "the false accusation that somehow the United States -- rather than the war itself -- is the source of tension in the gulf. Such statements are not helpful."

Informed in New York that an Iranian attack today had left a British tanker aflame in the gulf, the president responded, "I think that it's just symbolic of their barbarism," United Press International reported.

In his address, Reagan challenged the Soviet leadership to aid world peace by withdrawing its forces from Afghanistan, and hailed the agreement in principle reached last week by U.S. and Soviet negotiators to scrap intermediate-range nuclear missiles.

He promised enhanced efforts to reduce the superpowers' nuclear arsenals, and assailed the leftist Sandinista government of Nicaragua for "phony democratization."

The United States has been pressing for a second U.N. resolution that would impose an arms embargo against Iran if it refuses to accept the cease-fire. But U.S. officials acknowledge that other Security Council members have been reluctant to take this step.

After his speech, Reagan met for 15 minutes with U.N. Secretary General Javier Perez de Cuellar. A White House official told reporters that Reagan said during this meeting that he was "skeptical about Iran's intentions" to comply with a cease-fire. Perez de Cuellar, who recently visited Iran and Iraq, said he did know what position Iran would take.

Japanese Prime Minister Yasuhiro Nakasone, speaking to the General Assembly later in the day, favored mutual, voluntary restraint of weapons sales to Iran and Iraq rather than a formal arms embargo. Diplomatic sources said Nakasone, who has a close personal relationship with Reagan, made it clear he did not want a mandatory arms embargo.

The president also had a whirlwind round of meetings with other leaders, including Nakasone, Guatemalan President Vinicio Cerezo and Indonesian Prime Minister Mohammed Junejo.

The harshest language of Reagan's 31-minute address was reserved for the Sandinista leaders of Nicaragua, whom Reagan accused of enjoying "lives of privilege and luxury" while suppressing Nicaraguan freedom.

"This is why, despite a billion dollars in Soviet-bloc aid last year alone, despite the largest and best equipped army in Central America, you face a popular revolution at home," Reagan said. "It is why the democratic resistance is able to operate freely deep in your heartland."

The president praised the pending Central American peace plan, approved by five nations in the region, including Nicaragua. But he warned that "we will not, and the world community will not, accept phony democratization designed to mask the perpetuation of dictatorship."

Reagan then called upon the Sandinistas to restore complete freedom of speech, press and political activity and hold new elections. Unless this happens, he said, "democratization will be a fraud."

The president made no mention of the Nicaraguan government's authorization of the reopening of the opposition newspaper La Prensa, which White House spokesman Marlin Fitzwater called "a good first step." When Reagan was asked about it at a picture-taking session, he responded, "I hope that it is more than just show."

Reagan has scheduled a White House meeting Tuesday to discuss the Central American peace plan with its author, Costa Rican President Oscar Arias, who has been urging the administration to give the proposal a chance to work. A White House official said the president will tell Arias that continued support for the Nicaraguan contras is necessary while the peace plan is pending to "keep the pressure on the Sandinistas."

The Nicaraguan delegates listened to Reagan's attack upon their government without visible reaction. Soviet Foreign Minister Eduard Shevardnadze, heading his country's delegation, took frequent notes. Iranian and Afghan delegations were absent during Reagan's speech.

Victor Hugo Tinoco, deputy foreign minister of Nicaragua, said of Reagan's speech, "He's using the same old argument he was using before the Guatemalan agreement. Unfortunately, it means the Reagan administration is not changing its policy, and he is not giving the Central American peace treaty a chance."

U.S. officials were optimistic about arms control prospects last week in the aftermath of the U.S.-Soviet agreement in principle on a treaty to scrap medium- and shorter-range nuclear missiles and hold a superpower summit this fall in Washington. Reagan said he remained committed to his agreement with Soviet leader Mikhail Gorbachev to seek a 50 percent reduction in strategic nuclear arms.

But Reagan devoted much of his speech today to the theme that freedom is a worldwide necessity.

"Freedom in Nicaragua or Angola or Afghanistan or Cambodia or Eastern Europe or South Africa or anyplace else on the globe is not just an internal matter," he said. He quoted Soviet dissident Andrei Sakharov, a Nobel Peace Prize winner, that "disarmament and international security are inconceivable without an open society . . . and the right to travel and choose the country in which one wants to live."

Reagan also reiterated his intention of pursuing a missile defense system, his Strategic Defense Initiative (SDI), saying "SDI has greatly enhanced the prospects for real arms reduction" and is "a crucial part of our efforts to ensure a safer world and a more stable strategic balance."

And he praised the "courage . . . and people" of Pakistan for sheltering refugees from the Soviet invasion of Afghanistan "in the face of enormous pressure and intimidation."

U.S. officials said that some passages of the speech especially critical of the Soviet Union were toned down to make them less confrontational after U.S. and Soviet negotiators reached their agreement on an arms treaty last week.

"The changes were more of tone than substance, however," said one official. The official said the president wanted to make clear that last week's event had not removed the basic sources of U.S.-Soviet conflict, while at the same time indicating his willingness to act to improve the relationship.

Concluding his speech with an appeal for world peace, Reagan said, "I occasionally think how quickly our differences worldwide would vanish if we were facing an alien threat from outside this world. And yet, I ask you, is not an alien force already among us? What could be more alien to the universal aspirations of our peoples than war and the threat of war?"

Foreign ministers of the North Atlantic Treaty Organization met with Reagan shortly before he left New York and expressed "a real feeling of support and satisfaction" about the U.S.-Soviet move toward completion of an intermediate-range missile treaty, according to Secretary of State George P. Shultz. He said the allies' reaction was "uniformly enthusiastic."

The NATO secretary general, Lord Carrington, reporting on the same meeting, said the allies felt "a certain pride" that the NATO role had been instrumental in the agreement to remove Soviet missiles along with a lesser number of U.S. missiles from Europe.

Staff writer Don Oberdorfer and special correspondent Michael Berlin contributed to this report.