VENICE, JUNE 8 -- President Reagan sought to forge a unified position among the allies on nuclear arms control tonight as the economic summit opened amid continued allied concern over the risks of U.S. threats against Iran if it deploys the Silkworm antiship missile in the Persian Gulf.

British, French and Canadian officials here, while supporting the principle of free navigation in the gulf, appealed to the United States for a calmer approach to the region to avoid further belligerency by Iran and a wider confrontation involving the superpowers.

A senior British official said, "We favor reducing tension in the gulf, not increasing it." The appeals followed talk of U.S. retaliation or preemptive strikes against Iran if it deploys the Silkworm missiles near the Strait of Hormuz.

The leaders opened the 13th annual summit of the industrial democracies with a 3 1/2-hour dinner meeting at which they primarily discussed arms control, according to White House spokesman Marlin Fitzwater. He released a statement saying Reagan led a review of options in arms control negotiations with the Soviet Union in Geneva, and the leaders examined Soviet policy changes under Soviet leader Mikhail Gorbachev.

Earlier today, Reagan met with Japanese Prime Minister Yasuhiro Nakasone and announced the removal of some trade sanctions imposed against Japan six weeks ago. Reagan said he was releasing from tariffs $51 million in Japanese electronics goods, or 17 percent of those imports that had been penalized, saying this was "strictly proportional" to efforts by Japan to stop dumping semiconductors in Third World markets. {Details on Page A24.}

On another front, Chancellor Helmut Kohl told Reagan in a separate meeting that West Germany supports the proposal now being hammered out by the United States and Soviet Union to eliminate medium-range missiles in Europe. West German spokesman Friedhelm Ost said the two leaders are in "full agreement on arms control."

But White House chief of staff Howard H. Baker Jr. said that "the Germans are really uptight" about the proposal and "they have a long list of concerns." West German officials said Reagan told Kohl he expects a summit meeting with Gorbachev this autumn, where such an agreement could be signed.

In discussions about the impact of Gorbachev's reforms, "it was agreed that while Soviet policy changes seem significant, it is too early to fully assess the impact on the West," Fitzwater reported in a written statement. "All leaders agreed that they must wait for deeds, not words."

Fitzwater said Reagan outlined the U.S. negotiating position in Geneva and the first dinner meeting went 90 minutes longer than scheduled. Reagan has sought to use the summit to solidify allied support for a treaty limiting intermediate-range missiles.

At the dinner meeting, British Prime Minister Margaret Thatcher asked Reagan to explain whether the removal of some U.S. nuclear weapons from Europe in a possible deal with the Soviet Union would alter the American commitment to protect Europe with nuclear retaliation, diplomats reported. Reagan reassured Thatcher and other allied leaders that the U.S. commitment would not be weakened by the elimination of shorter- and medium-range nuclear missiles from Europe, they said.

Before the dinner, the various delegations were debating behind the scenes the implications of an increased superpower presence in the Persian Gulf following U.S. and Soviet plans to transport Kuwaiti oil under American and Soviet flags through hostile gulf waters. The issue is expected to come up among the leaders Tuesday.

Top U.S. officials continued to warn Iran today against deploying the Chinese-made Silkworm missiles. Baker said the missile "substantially escalates the danger to shipping" and said it would be in the "interests of peace" for Iran to "reconsider that action and to forgo" deployment.

The blunt talk from White House officials about the missile has clearly heightened anxieties among the allies about the risks of inadvertently drawing the superpowers into the Iran-Iraq conflict. French and Canadian spokesmen emphasized the need for the United States to "stay calm." A senior Canadian official warned against a "belligerent attitude" in referring to the stern rhetoric coming from the White House in recent days.

The allied leaders appeared ready to urge the United States to seek diplomatic rather than military solutions to the conflict by working through the U.N. Security Council and resolutions calling for a cease-fire. Canadian Prime Minister Brian Mulroney, who telephoned Reagan to express his general support for U.S. efforts to ensure freedom of navigation in the gulf, said he hoped the allies would support a strong U.N. resolution.

There was still debate over what kind of resolution could be approved by all summit participants and whether it is realistic to seek to impose mandatory sanctions against Iran if it continues to reject a cease-fire.

Chief of staff Baker and national security adviser Frank C. Carlucci said that the president is not seeking military aid but only diplomatic and symbolic support from the allies.

"Certainly for those that have ships in the gulf, we can coordinate our activities," Carlucci said. "But it's really not up to us to tell {Britain} you ought to have four ships instead of three ships." Carlucci said the United States was also seeking from the allies "assistance in dialogue with some of the gulf countries." Since colonial days, Britain has maintained close diplomatic and military contacts in the region.

Reagan was asked today whether he was bluffing Iran with indirect threats of retaliation or a preemptive strike. "I haven't bluffed once since I've been here," Reagan said. U.S. sources have been quoted in recent days as saying such a strike is one option being considered if the land-based missiles are deployed.

Reagan's 40-minute meeting with Kohl today was dominated by West Germany's concerns about an agreement to eliminate the intermediate-range missiles in Europe and set a limit of 100 for each side in Asia and the United States.

Kohl's coalition government officially endorsed the plan last week, on the condition that Bonn retain 72 Pershing IA missiles, which carry nuclear warheads under American control. The United States has supported this condition.