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.
Crew member Tom Urbik's parents speak of their concern
"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.
The operation to free Captain Richard Phillips, who was held captive in a lifeboat for five days, ended with three pirates being shot dead by marksmen from the USS Bainbridge on Sunday.
Somali pirate leaders - who have generally treated captives well in the hope of winning big ransom payouts - said they would avenge the deaths.
Map of pirate attacks
"No-one can deter us from protecting our waters from the enemy because we believe in dying for our land," Omar Dahir Idle told AP by telephone from the Somali coastal town of Harardhere.
Capt Phillips is flying back to the US on Wednesday.
US President Barack Obama promised on Monday to "halt the rise of piracy" in one of the world's busiest shipping lanes.
But in the 48 hours prior to the latest attack, four vessels have been seized in the same area:
* A Lebanese-owned cargo ship, the MV Sea Horse, was taken by gunmen in up to four skiffs
* A Greek-owned bulk carrier, the MV Irene, was also seized
* Two Egyptian fishing boats were held the previous day
Meanwhile, three Somali pirates who had taken French hostages are in custody in France, French prosecutors say.
The pirates were captured during a military operation to free hostages taken on the Tanit, a French yacht seized in the Gulf of Aden on 4 April.
The boat's French skipper and two other pirates were killed in the operation.
Lt Christensen said that although 15 countries had navies operating in the Gulf of Aden and off the coast of Somalia, the area was too large to prevent all attacks.
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"The area we patrol is more than one million square miles and the simple fact of the matter is we just can't be everywhere at once to prevent every attack of piracy," he said.
Our correspondent says it seems unlikely there will be any major increase in the military effort unless there is a spectacular hijacking involving the deaths of many Americans.
The reluctance to mount a major international naval operation in the area may also be down to the relatively small scale of the problem.
Last year, according to figures from the International Maritime Bureau, nearly 23,000 ships passed through the Gulf of Aden, but only 92 were hijacked.