An Iranian official, however, said it was not a serious incident, the state-run news agency IRNA reported.

U.S. military officials said the incident occurred early Sunday morning in the Strait of Hormuz, a narrow shipping channel leading in and out of the Persian Gulf.

They said that as the guided missile destroyer USS Hopper, the guided missile cruiser USS Port Royal and the guided-missile frigate USS Ingraham were entering the Persian Gulf, five Iranian boats approached them at high speed and swarmed them.

The Iranian boats made "threatening" moves toward the U.S. ships and in one case came within 200 yards of one of them, the U.S. officials said.

The U.S. Navy also received a radio transmission that officials believe came from the Iranian boats. The transmission said, "I am coming at you. You will explode in a couple of minutes," the U.S. military officials told CNN.

When the U.S. ships heard that radio transmission, they took up their gun positions and officers were "in the process" of giving the order to fire when the Iranians abruptly turned away, the U.S. officials said.

After the radio transmission, one of the Iranian boats dropped white boxes into the water in front of the U.S. ships, the officials said. It was not clear what was in the boxes, the officials said.

No shots were fired, and no one was injured.

A Pentagon spokesman characterized the incident as "perplexing" and "cause for real concern."

"Such actions are dangerous and could have quickly escalated into something much worse," said Geoff Morrell, Pentagon press secretary. "We see it as further evidence that Iran is unpredictable and remains a threat."

Iran's foreign ministry spokesman downplayed the incident, calling it "ordinary," IRNA reported. Mohammad Ali Hosseini said that similar incidents had occurred in the past between Iranian and American ships, and the issues were resolved as soon as the ships recognized each other, IRNA reported.

The Strait of Hormuz, which is in international waters, is near much of the world's oil supplies.

The White House urged Iran to refrain from "such provocative actions that could lead to a dangerous incident in the future," National Security Council spokesman Gordon Johndroe said.

U.S. State Department spokesman Sean McCormack said the United States probably would not make a formal protest to Iran about the incident.

McCormack said, "I can't speak to their rationale, their reasoning, their motivations."

Iran and the United States do not have diplomatic relations. Switzerland represents U.S. interests in Tehran, while Pakistan represents Iranian interests in Washington.

McCormack declined to comment further on the incident but said, "The U.S. will confront Iranian behavior where it seeks to do harm either to us or to our friends and allies in the region."

On Friday, the U.S. Navy announced the same team of naval ships had been searching in the Arabian Sea for a sailor missing for a day from the Hopper. The outcome of the search was not immediately known Monday.

In November the U.S. military reported that Iran's Revolutionary Guard Corps had taken command of Tehran's naval operations in the Persian Gulf.

The United States considers the Revolutionary Guard to be a major supporter of terrorist activity.

Tensions between Iran and the U.S. have increased over the last few years. The U.S. has concerns about Iran's nuclear program and has accused Iran of supplying weapons to insurgents in Iraq who target American forces.