WASHINGTON (Reuters) - Iranian boats aggressively approached three U.S. Naval ships in the Strait of Hormuz, a main shipping route for Gulf oil, at the weekend and threatened that the ships would "explode," U.S. officials said on Monday.

The Pentagon termed the action, involving five Iranian boats, "careless, reckless and potentially hostile" and said Tehran should provide an explanation, but Tehran dismissed U.S. concerns, saying it was a routine contact.

Vice Adm. Kevin Cosgriff, the commander of the U.S. Navy's Fifth Fleet, which is responsible for operations in the Gulf, said the Iranian boats moved aggressively toward the U.S. ships and their actions were "unduly provocative."

"The ships received a radio call that was threatening in nature, to the effect that they were closing on our ships and ... the U.S. ships would explode," Cosgriff told reporters at the Pentagon via videolink from his Bahrain headquarters.

The incident was the latest sign of tension between Washington and Tehran, at odds over a range of issues from Iran's nuclear program to U.S. allegations of Iranian support for terrorism and interference in Iraq.

U.S. President George W. Bush is due to travel to the Middle East this week on a trip he has said is partly aimed at countering Iranian influence.

Cosgriff said the U.S. Navy believed the Iranian boats belonged to the country's Revolutionary Guard and they were sometimes less than 500 yards (meters) from the U.S. ships.

In October, the United States designated the Revolutionary Guard Corps a proliferator of weapons of mass destruction and its elite Qods force a supporter of terrorism.

OIL PRICES ROSE

Oil prices briefly rose on the news about the confrontation as dealers weighed the threat to shipments along the key shipping route. Crude futures jumped 49 cents to $98.40 a barrel before slipping back.

In Tehran, the Iranian Foreign Ministry described the encounter as "ordinary" and said it had been resolved.

"This is an ordinary issue that happens for the two sides every once in a while and, after the identification of the two sides, the issue is resolved," Foreign Ministry spokesman Mohammad Ali Hosseini told Iran's official IRNA news agency.

An "informed source" from the naval force of Iran's Revolutionary Guards was quoted by Iranian state television as saying: "There were no out of the ordinary contacts between the Guards' naval force and American ships."

The source said three U.S. naval ships were asked by Guards' vessels "as usual" to identify themselves "which they did and they continued their path."

Pentagon spokesman Bryan Whitman said the three U.S. ships had been in international waters passing through the Strait of Hormuz on Sunday when they were approached by five fast boats, at least some of which were visibly armed.

"This is a serious incident," Whitman said. "Clearly this is something that deserves an explanation."

Other Pentagon officials, speaking on condition of anonymity, said after the Iranian threats a U.S. captain was in the process of ordering sailors to open fire when the Iranian boats moved away.

According to the officials, the radio transmission from one of the Iranian ships said: "I am coming at you. You will explode in a couple of minutes."

Cosgriff said the U.S. Navy was very mindful of the damage small craft could do to large ships. Al Qaeda militants killed 17 U.S. sailors when they rammed an explosives-laden boat into the side of the USS Cole, a destroyer, in Aden in 2000.

The incident took place about 0400 GMT Sunday, or late Saturday night in Washington, the officials said. Cosgriff said it was daylight with "decent visibility." The three U.S. ships were the USS Port Royal, USS Hopper and the USS Ingraham.

Last March, Iran seized 15 British sailors and marines in the Gulf and accused them of trespassing into Iranian waters. London maintained they were in Iraqi waters but the Britons were held for almost two weeks.

(Writing by Andrew Gray; Editing by David Storey)