The top U.S. military officer says the origin of threatening radio transmissions during an encounter between U.S. and Iranian boats in the Persian Gulf on Sunday is not as important as what the Iranian boats were doing. The statement came as the Pentagon released what it says is the full, unedited video of the half-hour incident. VOA's Al Pessin reports from the Pentagon.

At a news conference, the Chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, said it is difficult to know where the threatening radio transmissions came from. But he says the actions of the Iranian boats, approaching three U.S. Navy ships at high speed, maneuvering around them, not answering radio calls and dropping objects into the water, were threatening enough even without the transmissions.

And the admiral stressed that although no shots were fired during Sunday's incident, U.S. Navy ships will defend themselves if they have to. "We will defend ourselves and our ships, and we will do so with deadly force if need be," he said.

Admiral Mullen says the incident was the most provocative of several encounters the U.S. Navy has had with boats from Iran's Revolutionary Guard Force.

The latest controversy over the radio transmissions comes in the wake of a report in Friday's Washington Post, saying the U.S. Navy can not determine exactly where these threatening radio transmissions came from.

"I am coming to you. You will explode after a few minutes," the transmission said.

The voice was on an open channel for ship-to-ship communications.

On Friday, the Pentagon released additional video showing more of the incident, including a small object in the water. The U.S. Navy has said the Iranian boats dropped several objects into the water in the path of one of the American ships.

The new video shows several small boats approaching and moving around one of the American ships. Crew members of one ship report the sightings to another: "And control, 6-2, looks like two more high speed contacts bearing 3-1-0 from us, about four nauticals (7.4 kilometers) closing (on) your position."

One American officer is heard telling his gunners to be prepared to fire warning shots and then to direct fire at the Iranian boats, if necessary: "And obviously we'll give them warning shots before we do fire for effect, over. (second voice) OK, this is forward, roger that."

A moment later the order is given to accelerate to full speed: "All ahead flank. (second voice) All engines ahead flank. (third voice) All engines ahead flank. (first voice) OK, steer restricted maneuvering. (fourth voice) All engines ahead flank."

An officer is also heard ordering "Zebra Set": "Zebra Set, main deck and below."

A Pentagon spokesman says that is the order to close all the ship's doors and hatches to prepare to take damage.

Although the American warships are much larger than the Iranian boats, it was a small boat similar to these, laden with explosives, that caused major damage to the USS Cole in Yemen in 2000, killing 17 American sailors.

The Iranian boats turned away before any shots were fired.

The new video also appears to depict an earlier encounter with Iranian boats in which the Iranians identify themselves as Coast Guard and do not take any aggressive action. That contact is similar to a competing video released by Iran.

Admiral Mullen says the action by the Revolutionary Guard boats appears to be part of a shift by Iran to more aggressive tactics and more involvement in the Gulf by the Guard boats, rather than Iranian Navy vessels, which operate more in accordance with normal maritime practices. The admiral says the Guard boats appear to be probing to see what they can get away with.

"Clearly, when you go through this kind of incident there's going to be testing. We're going to learn about each other. And there's no taking that off the table. So I'm sure we learned about them and they learned about us," he said.

Admiral Mullen says Iran should not confuse the U.S. Navy's restraint in this situation with a lack of capability or willingness to fire if necessary to defend U.S. or allied forces.