(CNN) -- The Liberty Sun, a U.S.-flagged cargo ship bound for Mombasa, Kenya, was attacked Tuesday by Somali pirates, according to a NATO source with direct knowledge of the matter.

Pirates attacked The Liberty Sun, a U.S.-flagged cargo ship, but were unable to board.

"The pirates fired rocket-propelled grenades and automatic weapons at the vessel, which sustained damage," said a statement from New York-based Liberty Maritime Corporation, which owns the vessel.

The ship was carrying U.S. food aid for African nations, the statement said.

The pirates never made it onto the ship and the vessel is now being escorted by a coalition ship, still bound for Mombasa, officials said.

Two senior defense officials said the Liberty Sun was being escorted by the guided missile destroyer USS Bainbridge. It is the ship carrying Richard Phillips, the captain of the container ship Maersk Alabama, which was hijacked last week. Phillips spent days as a hostage of the pirates before being rescued Sunday.

Katy Urbik of Wheaton, Illinois, said her son, Thomas, was aboard the Liberty Sun at the time of the attack. She shared the e-mails he sent as the ship came under fire.

"We are under attack by pirates, we are being hit by rockets. Also bullets," said one e-mail sent Tuesday afternoon. "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.

"Navy is on the way and helos and ships are coming. I'll try to send you another message soon. [G]ot to go now. I love you mom and dad and all my brothers and family."

"My heart stopped after I realized there wasn't going to be a 'just kidding' after his comment," Katy Urbik said.

About 1½ hours later, Thomas Urbik sent another e-mail to his mother, which said, "The navy has showed up in full force and we are now under military escort ... all is well. I love you all and thank you for the prayers."

In an e-mail only hours before the attack, Urbik's son tried to assure his mother that his crew was safe and taking precautions.

"Don't worry too much. I am fine and we are being well monitored by the U.S. Navy, who is demanding we send them a report every six hours on our position and status," Thomas Ubrik's e-mail said. He added, "We in fact are going to be the second American ship to arrive into Mombasa after the Maersk Alabama. It should be interesting to say the least. ... We have had several drills to prepare ourselves to secure ourselves in the engine room. [W]e can do it pretty quick by now."

The company said the ship had dropped off food aid last week at a Sudanese port and the ship was going around the Horn of Africa to reach Kenya when it came under attack. However, the exact location of the attack remained unclear.

Earlier Tuesday, pirates off the coast of Somalia seized two freighters, proving they remain a force to contend with just days after the U.S. Navy dramatically rescued an American captain held by other pirates.

First, pirates in the Gulf of Aden on Tuesday hijacked the MV Irene EM, a 35,000-ton Greek-owned bulk carrier, according to a NATO spokesman and the European Union's Maritime Security Center.

The crew of the Greek carrier was thought to be unhurt and ships have been warned to stay clear of the area for fear of further attack, the Security Center said.

Later Tuesday, pirates on four skiffs seized the 5,000-ton MV Sea Horse, a Lebanese-owned and Togo-flagged vessel, said Cmdr. Chris Davies of NATO's Maritime Component Command Headquarters in Northwood, England.

Details about the ship and its crew weren't immediately available.

NATO has an ongoing anti-piracy mission off Somalia called Operation Allied Protector. The mission involves four ships covering more than a million square miles, Davies said.

A U.S.-led international naval task force, Combined Task Force-151, is also patrolling in the region.

Tuesday's hijackings came two days after sharpshooters from the U.S. Navy SEALs killed three pirates who had been holding Phillips hostage on the water for days.

Phillips had offered himself as a hostage when pirates attacked the Alabama on Wednesday, officials said. The ship had been on its way to deliver aid to Mombasa, Kenya.

A fourth pirate had been aboard Bainbridge when the shootings occurred and was taken into custody. Watch the tough tactics the Navy uses »

The incident follows four freighters being seized over the past two days by pirates off the Somalian coast, proving they remain a force to contend with.

Pirates on Monday hijacked two Egyptian fishing boats carrying a total of between 18 and 24 people, the Egyptian Information Ministry told CNN.

The Egyptian Foreign Ministry is working to end the hijacking, the ministry said.

Egyptian boats are known to use Somali waters illegally for fishing, taking advantage of the lawless state of the country and the lack of enforcement of its maritime boundaries.

Those who have tracked pirate activity in Somalia say it started in the 1980s, when the pirates claimed they were trying to stop the rampant illegal fishing and dumping that continues to this day off the Somali coast.

Piracy accelerated after the fall of the Somali government in the early 1990s and began to flourish after shipping companies started paying ransoms. Those payments started out being in the tens of thousands of dollars and have since climbed into the millions.

Some experts say companies are simply making the problem worse by paying the pirates.

CNN's Mike Mount, Barbara Starr and David McKenzie contributed to this report.

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