WASHINGTON — Iranian boats harassed and provoked three American Navy ships in the strategic Strait of Hormuz, threatening to explode the vessels, American officials said today.

In the most serious such incident in years, American forces were on the verge of firing on the Iranian boats during the incident early yesterday, when the boats — believed to be from the Iranian Revolutionary Guard’s navy — turned and moved away, a Pentagon official said.

A Defense Department spokesman, Bryan Whitman, called it a “serious incident.” Another American official, speaking on condition of anonymity, called it “the most serious provocation of this sort that we’ve seen yet.”

Iran’s Foreign Ministry said today the confrontation was “something normal” and was resolved, suggesting the Iranian boats had not recognized the American vessels. A National Security Council spokesman, Gordon Johndroe, said the Bush administration urges Iranians “to refrain from such provocative actions that could lead to a dangerous incident in the future.”

The incident raised new tensions between Washington and Tehran as President Bush prepared for his first major trip to the Middle East.

A statement issued by the U.S. Navy’s 5th Fleet in Bahrain said the incident occurred at about 8 a.m. local time yesterday as Navy cruiser USS Port Royal, destroyer USS Hopper, and frigate USS Ingraham were on their way into the Persian Gulf and passing through the strait — a major oil shipping route.

Five small boats began charging the American ships, dropping boxes in the water in front of the ships and forcing the American ships to take evasive maneuvers, the Pentagon official said. The boxes floated by, and officials said they didn’t now what was in them because American sailors didn’t pick them up.

There were no injuries but the official said there could have been, because the Iranian boats turned away “literally at the very moment that U.S. forces were preparing to open fire” in self defense.

The official, who asked to speak on grounds of anonymity because he wasn’t authorized to discuss the issue publicly, said he didn’t have the precise transcript of communications that passed between the two forces, but said the Iranians radioed something like “we’re coming at you and you’ll explode in a couple minutes.”

At the State Department, a spokesman, Sean McCormack, said he was not aware of any plans to lodge a formal protest.

“Without specific reference to this incident in the Strait of Hormuz, the United States will confront Iranian behavior where it seeks to do harm either to us or to our friends and allies in the region,” Mr. McCormack told reporters. “There is wide support for that within the region and certainly that’s not going to change.”

Mr. Whitman said the Pentagon will work with State and National Security Council officials to determine “the appropriate way to address this with the Iranian government.”

But an Iranian Foreign Ministry spokesman, Mohammed Ali Hosseini, played down the incident, suggesting it was an issue of misidentification. He did not comment on the American claims of the Iranian boats’ actions.

“That is something normal that takes place every now and then for each party, and it (the problem) is settled after identification of the two parties,” he told the state news agency IRNA.

The incident was “similar to past ones” that were resolved “once the two sides recognized each other,” he said.

An Iranian Revolutionary Guard official also described the incident as nothing unusual.

“No unusual confrontation has taken place between the Guard’s patrol vessels and U.S. ships,” state-run television quoted the official as saying. The official was speaking on customary condition of anonymity. The Guard official said the Guard’s vessels were conducting normal patrols in the Strait of Hormuz when they saw three American ships enter the waters of the region.

“The Guard’s navy vessels, as usual, asked the ships to identify themselves and they did so and continued their path,” the TV quoted the official as saying.

At the Pentagon, Mr. Whitman said the American vessels were in international waters, making a normal transit into the Gulf. He said the Iranian boats were operating at “distances and speeds that showed reckless and dangerous intent — reckless, dangerous, and potentially hostile intent.”

The episode lasted 15 to 20 minutes, Mr. Whitman said, but he wouldn’t say whether officials know for certain whether the were vessels were Iranian Revolutionary Guard or regular Iranian navy. The Revolutionary Guard forces have been known to be more aggressive than the regular navy.

“At least some were visibly armed. Small Iranian fastboats made some aggressive maneuvers against our vessels and indicated some hostile intent,” Mr. Whitman said.

Historical tensions between the two nations have increased in recent years over Washington’s charge that Tehran has been developing nuclear weapons and supplying and training Iraqi insurgents using roadside bombs — the no. 1 killer of American troops in Iraq.

At about this time last year, Mr. Bush announced he was sending a second aircraft carrier to the Gulf region in a show of force against Iran. The U.S. Navy quietly scaled back to one carrier group several months later. But while the two were there, they staged two major exercises off Iran’s coast.

As one of the world’s most vital chokepoints for oil shipping, the 30-mile-wide Hormuz strait has been the subject of previous armed confrontations between America and Iran, most notably during the eight-year Iraq-Iran war of the 1980s.