Robertson told Al Jazeera the confrontation should not be viewed as an incident.
She said: "We look at this as an event, an encounter, but not an incident.
"Our other ships in the area are continuing to operate as needed for maritime security."
Defence officials initially suspected the boats were Iranian.
But Stephanie Murdoch, another spokeswoman for the Fifth Fleet, denied the claims.
She said: "We cannot speculate on who they are. We just don't know. "We have no proof of who they were."
In Tehran, an Iranian navy source denied that any confrontation had occurred with a US ship in the Gulf.
But the source, quoted by a journalist for Iran's state-owned Arabic Al-Alam TV channel, said any shooting that may have occurred could have targeted a non-Iranian vessel.
Relations between Washington and Tehran are tense over Iran's nuclear programme.
Hostile rhetoric and close encounters in the Gulf have fuelled speculation that the US may be planning some sort of military action against Tehran.
Continued accusations of Iranian involvement in threats against its ships have also contributed to the tension.
In January, the US said five small Iranian speed boats aggressively approached three navy ships in the Strait of Hormuz, an oil shipping route.
During the confrontation, a radio message was received warning the US ships they could explode within minutes.
But Iran said its boats were simply trying to identify the vessels and maritime experts said the threatening message may have come not from the Iranian boats but from someone interfering with the radio transmission.