This image released by the U.S. Navy on Tuesday and shot Sunday from the bridge of the destroyer USS Hopper, shows a small blue boat, alleged to be Iranian, purportedly racing near the wake of U.S. Navy ships in the Persian Gulf.

Washington ? Recent clashes between Iranian and U.S. Navy forces in the Persian Gulf reflect Iran’s shifted military strategy to use its Revolutionary Guard’s fast boats more aggressively in the region, the top U.S. military officer said Friday.

In a confrontation Sunday – captured on a 36-minute video the Pentagon made public Friday – military officials said boxes were thrown into the water by the Iranians, triggering concerns about potential mine threats. And in an incident last month, a U.S. ship fired warning shots at a rapidly approaching Iranian boat.

While there are lingering questions about the origin of menacing verbal threats heard during the confrontation Sunday in the Strait of Hormuz, Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said Friday that the clash was the most “provocative and dramatic” he has seen.

“The incident ought to remind us all just how real is the threat posed by Iran and just how ready we are to meet that threat if it comes to it,” Mullen said.

Iran denied its boats threatened the U.S. vessels and accused Washington of fabricating its video. A five-minute video released by Iran shows a man speaking into a handheld radio, with three U.S. ships floating in the distance. That footage did not show any Iranian boats approaching the U.S. vessels nor any provocation.

Adm. William J. Fallon, the top U.S. military commander in the Mideast, told The Associated Press that Iran runs the risk of triggering an unintended conflict if its boats continue to harass U.S. warships in the strategic waterway.

Both Mullen and Fallon said they could not tell if the verbal threats heard in a Pentagon-released audio tape came from the Iranian boats. In the recording, a man with an accent can be heard warning in English: “I am coming to you” and then, “You will explode after … minutes.”

The United States has lodged a formal diplomatic protest with Iran over Sunday’s incident, underscoring the increasing tension between the two countries.

Also Friday, the Navy for the first time described the December encounters in detail.

The USS Whidbey Island fired the warning shots on Dec. 19 in response to a small Iranian boat that was rapidly approaching, said a Navy official, who spoke on condition of anonymity because of the sensitivity of the issue.

Three days later, the USS Carr encountered three small Iranian craft, two of which were armed, said the official. The USS Carr did not fire warning shots but sent warning blasts on the ship’s whistle, which caused the boats to turn around.

Sunday’s incident, in which five Iranian fast boats swarmed a convoy of three U.S. warships, caused the most concern in the Pentagon and also heightened interest in the grainy video shot by a crew member on the destroyer USS Hopper.

The video, which Pentagon spokesman Bryan Whitman said was unedited, showed U.S. sailors answering radio calls from the Iranians and monitoring the five fast boats. For much of the time, the camera followed one or more of the fast boats moving around the U.S. ships. And at one point, it zoomed in on an object floating in the water – which Navy officials said was one of the boxes the Iranians were seen throwing overboard.

As the incident escalated, sailors on the Hopper can be heard growing more agitated about how fast and close the small boats were moving near the ships. They can also be heard referring to the fact that the U.S. ship had gone on high alert, in which all of the hatches and doors on the main deck and below were closed to prepare the ship for potential damage.

The verbal threats cannot be heard in the video because they were in a separate audio recording released earlier in the week.