A Navy destroyer carrying Captain Richard Phillips after he was rescued from armed Somali pirates this week had to divert to rescue another ship that was under attack yesterday.

The USS Bainbridge was called on to help a US cargo ship, the MV Liberty Sun, which was under attack by Somali pirates using rocket-propelled grenades and automatic weapons.

But by the time the destroyer reached the MV Liberty Sun the pirates had fled, giving up their attempt to hijack the ship or kidnap any of its crew.

Tuesday’s brazen midday attack on the Liberty Sun in international waters off the African coast is further evidence that Somali pirates are back to business as usual.

Pirates have seized four other ships with 60 hostages since US snipers killed three gunmen holding Captain Phillips after he was kidnapped from his container ship. A Greek-owned freighter, the MV Irene, was also taken in the Gulf of Aden with more than 20 crew yesterday. Pirates in as many as four speedboats seized the MV Sea Horse, a 5,000-tonne cargo ship flying the Togolese flag, soon afterwards.

The Liberty Sun's American crew were not injured but the vessel sustained unspecified damage in the attack, its owner, the Liberty Maritime Corp. said in a statement last night.

Earlier, the mother of a crew member of the Liberty Sun told of e-mails her son had sent while the crew barricaded themselves in the engine room when the ship came under attack.

“We are under attack by pirates, we are being hit by rockets. Also bullets,” Thomas Urbik, 26, wrote in an e-mail on Tuesday. “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.”

It was not immediately clear what happened next, but Mr Urbik sent a follow-up e-mail “that said he was safe and they had a naval escort taking them in”, his mother, Katy Urbik, said.

The attack on the MV Liberty Sun occurred hours after President Obama promised to halt the rise of piracy off the Horn of Africa.

Navy Captain Jack Hanzlik said that the USS Bainbridge responded to the attack but the pirates had departed by the time it arrived some six hours later.

Bainbridge is the destroyer from which snipers killed the three pirates who held Captain Phillips aboard a drifting lifeboat for five days. The destroyer was carrying Captain Phillips, who is due to return home to the US today, to Kenya when called to help the Liberty Sun.

The Liberty Sun, with its crew of about 20 Americans, was carrying humanitarian aid to Mombassa, Kenya, Captain Hanzlik said. The ship continued on to Kenya after the attack under a Naval escort.

The pirates have vowed vengeance for five of their colleagues who have been killed by US and French forces in two hostage rescues since Friday.

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