Iran has described the incident as an "ordinary occurrence"
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Iranian speedboats approached US warships in two previously undisclosed incidents in the Strait of Hormuz in December, a US Navy official has said.
The USS Whidbey Island fired warning shots during one of the encounters on 19 December, said the official, who spoke on condition of anonymity.
He also described a third stand-off on 6 January - that sparked US protests - as the most serious incident.
Iran has accused the US of faking its video of the most recent encounter.
'Radio threat'
The US official said the USS Whidbey Island had to act in response to an Iranian boat that was rapidly approaching the amphibious warship on 19 December.
"One small [Iranian] craft was coming toward it, and it stopped after the Whidbey Island fired warning shots," the official was quoted as saying by the Associated Press.
In the second incident on 22 December, the USS Carr, a guided missile frigate, encountered three small Iranian boats, said the official.
He said that the US vessel blew warning whistles, causing the boats to turn around.
His comments came shortly after Washington had sent Iran a formal protest over the stand-off on 6 January.
Pentagon officials have said the speedboats came within about 200m (650ft) of the US vessels.
The US military have said video and audio that it released confirmed its allegation that Iranian speedboats harassed US vessels and threatened to blow them up in a radio communication.
However, senior US navy sources later told the BBC that the alleged threat to blow up the warships "may not have come" from Iranian speedboats.
Iran has rejected Washington's allegations, issuing its own video footage of the incident.
The confrontation has further inflamed long-running tensions between Iran and the United States.