Iranian speedboats interfered in the passage of US warships in the Gulf's Strait of Hormuz on two previously undisclosed occasions, the US has said.
The USS Whidbey Island and USS Carr had to issue warnings to Iranian vessels in December, said a Pentagon official, who spoke on condition of anonymity.
The allegations follow a third stand-off on 6 January, of which the US and Iran gave conflicting accounts.
Iran accused the US of faking footage of its vessels "harassing" US ships.
Iran, which described the encounter as routine and ordinary, aired its own video that it said showed the incident between Revolutionary Guards patrols and US warships.
There was no sign of threatening behaviour by the Iranian boats, as alleged by US officials who said their actions were dangerous and provocative.
Iran has so far not responded to the latest claims by the US.
A Pentagon official said the USS Whidbey Island, an amphibious warship, had been forced to fire warning shots after an Iranian boat approached it at high speed on 19 December.
In another incident on 22 December, the USS Carr, a guided missile frigate, encountered three small Iranian boats, the same official said.
HOW THE ROW UNFOLDED
6 Jan: Date of alleged incident in Strait of Hormuz
7 Jan: Washington makes public allegation against Iran
8 Jan: US releases video and audio to back its claim
9 Jan: Iran says US video was fabricated
10 Jan: Iran releases footage, contradicting US account
Senior US Navy officials voice doubts over origin of threat
11 Jan: US airs what it says is unedited video of incident
The US vessel blew warning whistles, causing the boats to turn around, he said.
But US officials have said the confrontation on 6 January was the most serious to date.
The US alleges that five speedboats came within about 200m (650ft) of two US vessels transiting through the Strait of Hormuz.
A day after making the claim, the US military released five minutes of video and audio, which appeared to back its account of the stand-off.
At one point during the audio recording a heavily-accented voice in English can be heard radioing: "I am coming to you. You will explode after a few minutes."
Iran said it was a fabrication. Senior US Navy sources later told the BBC that the alleged threat to blow up the warships "may not have come" from Iranian speedboats.
On Friday, the US defence department aired what it said was the unedited video.
The footage, which lasts more than 30 minutes, does not appear to show anything very different from what was seen in the extract already released nor does it shed any light on the origin of the voice heard on tape making threats.
The US said that in any case the Iranian speedboats had acted aggressively. Iran's version is that this was a routine check by its sailors.
Tensions between Iran and the US have diminished recently following the US National Intelligence Estimate that Iran is not building a nuclear weapon.
But BBC world affairs correspondent Paul Reynolds says there are still serious points of potential conflict between the two.
Iran is determined to exert its influence in what it calls the Persian Gulf and the US is intent on maintaining strong naval forces in international waters there and in the waters of its Gulf Arab allies, our correspondent notes.