Pirates have hijacked a tanker off the coast of Somalia, maritime officials say, days after several pirates were killed by US and French forces. In the latest in a string of attacks by pirates seeking ransom payments, a freighter named as the MV Irene was taken in the Gulf of Aden. Officials were unable to confirm the nationalities of the crew on board. It comes after US and French troops managed to free several hostages by opening fire on two groups of pirates. After a lull earlier this year, pirates have stepped up their attacks off Somalia's coast in recent weeks. Officials from the EU Maritime Security Centre said the Irene, a 35,000-tonne Greek-owned vessel, was seized overnight. The centre said the 22-man crew is believed to be unhurt and shipping has been warned to stay clear of the area for fear of further attacks. Power vacuum In a high profile hijack in the Indian Ocean off Somalia last week, a group of attackers attempted to seize a US-flagged ship - believed to be the first time a US vessel has been hijacked in two centuries. The pirates eventually fled the ship, taking its American captain hostage. But after a stand-off with a US warship, three pirates were killed by US snipers and the captain was freed. US President Barack Obama has now promised to "halt the rise of piracy" in one of the world's busiest shipping lanes. "I want to be very clear that we are resolved to halt the rise of piracy in that region," he said following the operation to free Richard Phillips, captain of the Maersk Alabama. "To achieve that goal we're going to have to continue to work with our partners to prevent future attacks." The US operation came shortly after French forces killed two pirates and captured three more while freeing a yacht with five crew. The yacht's owner was also killed in the gunfight. 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). Efforts to stop the pirates have so far had only limited success, with international naval patrols struggling to cover the vast areas of ocean where the gangs operate. Source: BBC