Washington/Nairobi - A US-flagged cargo ship, The Liberty Sun, was steaming to the Kenyan port of Mombasa under US Navy escort Wednesday after escaping an attempted hijacking by Somali pirates. "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, the owners of the ship.

An email message from a crewman to his mother after the incident, which happened Tuesday, said the crew of the vessel were safe and under escort.

Senior defence officials were quoted as saying the Liberty Sun was being escorted by the guided missile destroyer USS Bainbridge.

The Bainbridge was also carrying Richard Phillips, the captain of the container ship Maersk Alabama, which was hijacked last Wednesday.

The US crew of the Maersk Alabama fought off pirates who boarded the ship, but Phillips was held hostage for five days on a lifeboat.

He was freed Sunday when five US Navy snipers killed the three pirates holding him.

Phillips is due arrive in Mombasa on Wednesday, from where he will fly back to the US with his crew.

On Friday, the captain of a French yacht and two pirates died when French naval forces stormed the boat, freeing four hostages.

Pirate gangs have remained undaunted by the French and US actions, however.

Since Monday, pirates have seized the Greek MV Irene EM, the Lebanese owned MV Sea Horse and two Egyptian fishing boats.

Pirate activity has picked up in recent weeks after a lull due to bad weather.

Over 20 ships have been attacked in the last three weeks. Nine of those ships are in pirate hands, bringing the total number of ships being held to 17. Almost 300 crew members are being held hostage.

Piracy experts have said they do not expect the attacks to deter pirates from seeking multi-million-dollar ransoms.

On the contrary, there are now fears that pirates will take more extreme action, increasing the chance of further hostage deaths.

Pirate gangs have already vowed to avenge their dead colleagues by targeting US and French citizens.

In 2008, pirates seized more than 40 vessels in and around the Gulf of Aden and collected tens of millions of dollars in ransoms, prompting the international community to send warships to the region.

Around 15 warships from the European Union, a US-led coalition task force and individual countries such as Russia, India and China patrol an area of about 2.85 million square kilometres.

The presence of the warships has appeared to have had little effect, and observers say that only by tackling insecurity and poverty in Somalia will piracy finally be halted.

Somalia has been without a functioning government since the 1991 ouster of dictator Mohamed Siad Barre.