U.S lodges diplomatic protest against Iran

The incident was another sign of tension between the United States and Iran, at odds over a range of issues including Tehran's nuclear program and its alleged role in Iraq.

The Strait of Hormuz is the most prominent potential "choke point" for crude oil flows, handling 17 million barrels per day, or two-fifths of globally traded oil.

Close to opening fire
U.S. officials have said U.S. sailors were close to opening fire Sunday before the Iranian boats moved away. They have said they believe the boats came from Iran's Revolutionary Guard Corps.

"I'd much rather prevent a war than fight one," Mullen said. "We'd all prefer Iran to take a more productive, positive role in the region. And I support the use of economic and diplomatic measures to help bring that about."

"But our own military restraint in dealing with that problem should in turn never be confused for a lack of capability."

Some U.S. commentators have suggested the U.S. sailors should have opened fire and their restraint will be interpreted by Iran as a sign of weakness. But Mullen, a former head of the U.S. Navy, said the crews' actions had been "exactly right."

Sides release videos, conflicting accounts
Both the United States and Iran have issued video footage to support their conflicting accounts of the incident.

The United States has also released the recording of a message it says was received by one of the U.S. ships. "You will explode after... minutes," a heavily accented voice on the recording says.

U.S. officials initially said the audio was believed to have come from one of the Iranian boats but they have since said they are not certain of its exact origin.

"I can't shed any light as far as the radio transmission is concerned," Mullen said. "If you're out there on the bridge, it's hard to tell where (radio transmissions are) coming from."

Tehran's strategy shift
The recent confrontation in the Persian Gulf reflects a strategy shift by Tehran to use its Revolutionary Guard's fast boats more aggressively in the region, Mullen said.

Mullen said the U.S. has been focused "for several years" on this shift to greater use of small, fast boats by the Iranian Revolutionary Guard Corps, which has taken over patrols in the Gulf from Tehran's regular navy.

"It's clearly strategically where the Iranian military has gone," said Mullen. "There's a projection they were going to do that over a number of years ... That was a big concern to me because of the history and the background with the (Revolutionary Guard). This fits that mold, as far as I was concerned."