US military chiefs are bolstering their forces around the Horn of Africa to help tackle Somali pirates holding a US captain hostage in a drifting lifeboat. Gen David Petraeus said reinforcements would arrive within 24 to 48 hours and that a warship already in the area was "literally right near" the lifeboat. FBI experts are helping negotiate the release of Capt Richard Phillips. He was captured in a struggle on his ship, Maersk Alabama. Pirates seized the ship but the crew fought them off. The vessel has now set sail for the Kenyan port of Mombasa under armed guard. 'Nothing more than criminals' The announcement of reinforcements came amid rising concern over the fate of Capt Phillips. "The safe return of the captain is the top priority," US Defence Secretary Robert Gates told reporters in Washington. The cruise-missile carrying USS Bainbridge was sent to the scene in a move analysts say will strengthen the hand of US negotiators. Secretary of State Hillary Clinton described the attackers as "nothing more than criminals". "We are bringing to bear a number of our assets, including naval and FBI, in order to resolve the hostage situation and bring the pirates to justice," she said. Gen Petraeus said the additional US forces would "ensure that we have all the capability that might be needed over the course of the coming days". There are thought to be four pirates with Capt Phillips in the lifeboat, which has reportedly run out of fuel, and it is not yet clear what they are demanding. Capt Phillips has a radio and contacted the navy and crew of his ship to say he was unharmed, the Maersk shipping company said in a statement. A Somali who claimed to be a pirate leader told AFP news agency he would send support to the other pirates. "We are planning to reinforce our colleagues who told us that a navy ship was closing in on them and I hope the matter will soon be solved," a man AFP named as Abdi Garad said by phone from the Somali pirate lair of Eyl. It was not possible to verify his identity. Destabilisation concern Analysts have said negotiations could be lengthy, with the pirates likely to want a hefty ransom for the captain as well as compensation for a boat that was wrecked in the attack. The cargo ship, carrying food aid destined for Somalia and Uganda, was seized about 500km (311 miles) off Somalia's coast in the early hours of Wednesday. After a long struggle, the crew members regained control of the ship. It is thought that Capt Phillips offered himself as a hostage in order to save his crew. Somalia has been without an effective government since 1991, fuelling the lawlessness which has allowed the pirates to thrive. Pirates typically hold the ships and crews until large ransoms are paid by the shipping companies - last year the firms handed over about $80m (£54m). After a lull earlier this year, the Maersk Alabama was the sixth ship hijacked off Somalia in the past week. The UN's Somalia envoy, Ahmedou Ould-Abdallah, told Reuters that piracy was threatening to destabilise the region. Source: BBC