A maritime watchdog hailed the "robust response" by US and French forces to rescue their citizens from Somali pirates but cautioned it may spark retaliatory attacks on seafarers.

US Navy snipers ended a tense high-seas drama on Sunday evening when they killed three pirates and rescued Captain Richard Phillips, who had commanded the Maersk Alabama cargo ship and was held captive for five days in a lifeboat.

In a separate action, French commandos Friday stormed a yacht held by Somali pirates in an operation that left one French hostage and two gunmen dead.

"We support the robust response against the pirates," said Noel Choong, head of the International Maritime Bureau (IMB) piracy reporting centre in Kuala Lumpur.

"It is the right of the flag state to take these military operations. We are certain that the military would have considered all other options before taking the final decision," he added.

But Choong said attacks would remain rampant in the pirate-infested waters off the Horn of Africa because there was no single uniform plan to fight the pirates.

"The military actions by the French and US forces will not deter piracy because not all countries are taking similar measures," he said.

"Next time we get American citizens..."

In previous hijackings, many countries and ship owners have paid a ransom to secure the release of their vessels and sailors.

Choong also warned that Somali pirates may launch retaliatory attacks on merchant ships and seafarers for the death of their gang members.

"It may spark retaliatory action by the pirates. It may increase violence against the ships and crew members," he said.

A Somali pirate chief has already threatened to target Americans in revenge for the rescue of the US captain.

"We will intensify our attacks even reaching very far away from Somalia waters, and next time we get American citizens... they (should) expect no mercy from us," Abdi Garad said by phone from the pirate lair of Eyl.

Choong also warned that the arming of seafarers would simply escalate violence at sea despite a top US navy chief saying it would deter piracy.

"We do not support arming the untrained crew or placing mercenary soldiers on board the ship because it will escalate violence and create a lot of legal complications if someone is wrongly shot or killed," he said.

The commander of US naval forces in the region said US shipping companies need to provide armed guards for vessels threatened by pirates.

"You need two things to have a successful piracy attack. You need pirates that are seeking monetary gain and you also need a ship that's able to get pirated," Vice Admiral William Gortney told CNN on Monday.

Choong said Somali pirates were holding 13 ships and 228 seafarers. Since January there has been 74 attacks compared to 111 in 2008.