Pirates have used rocket-propelled grenades and automatic weapons to attack another US merchant ship off the coast of Somalia.
The pirates damaged the Liberty Sun, which was carrying a cargo of food aid, but were not able to board it.
The ship asked for assistance from the American warship involved in the rescue of a US captain seized last week.
Pirates have vowed to avenge the deaths of those killed in recent rescue operations by US and French forces.
BBC security correspondent Rob Watson says that although it is not clear if the latest attack was intended as the promised revenge, it shows that pirates have not been deterred by military operations.
Despite renewed US calls to quell piracy, four more vessels have been successfully seized over the past two days.Details of attack
Owners of the Liberty Sun and the US military confirmed reports of the latest, failed attack, which took place on Tuesday at midday local time.
Some details were revealed in an e-mail from one of the crewmen to his mother at her home in Illinois, AP news agency reported.
"We are under attack by pirates, we are being hit by rockets, also bullets," Thomas Urbik, 26, told his mother, Katy.
"We are barricaded in the engine room and so far no-one is hurt. (A) rocket penetrated the bulkhead but the hole is small. Small fire, too, but put out."
Lt Nathan Christensen, of the US Fifth Fleet in Bahrain, told the BBC that the ship had taken "evasive manoeuvre action" which had prevented the pirates from boarding.
The Liberty Sun had been en route to Mombasa from Houston, Texas, when the attack took place.
After coming under fire, the ship immediately requested assistance from the USS Bainbridge, said owners Liberty Maritime Corp in a statement.
The navy destroyer arrived some hours later, by which time the pirates had gone.
"We are grateful and pleased that no-one was injured and the crew and the ship are safe," said the Liberty Maritime Corp statement.
The ship did sustain some damage, it said, but was able to resume its journey to Mombasa.