NAIROBI (Agencies) - Somali pirates captured four ships and took more than 60 crew members hostage in a brazen hijacking spree, brushing off their losses from deadly rescue operations and throwing down the gauntlet to US President Barack Obama after he pledged to curb piracy. The Somali pirates seized two more ships on Tuesday. It brought to four the number of vessels taken since the US navy operation on Sunday which saved an American skipper but led to the deaths of three pirates, upping the stakes in the dangerous waters off the Horn of Africa. The MV Irena, a 35,000-tonne Greek-operated merchant vessel flagged in Saint-Vincent and the Grenadines, was hijacked in the Gulf of Aden, the European Union's naval mission in the area said. Its 22 Filipino crew is believed to be safe. Hours later a Nato spokeswoman said a second freighter, flying a Togolese flag, had been seized by pirates off the Horn of Africa, the 10th hijacking in the Gulf of Aden and Indian Ocean since the start of the month. "I can confirm that a second cargo ship, the Sea Horse has been seized," said spokeswoman Shona Lowe from Nato's Northwood maritime command centre in England. She could not provide details on the numbers or nationalities that had been aboard the ship nor how many remained in danger. The pirates attacked the vessel "on three or four skiffs," she said. Andrew Mwangura of the East Africa Seafarers Assistance Programme said the owners of the Greek ship had another vessel, the 64,000-tonne Maltese-flagged Panamax Anna, attacked in the area on Monday. The vessel "was attacked by six pirates in a boat in the Gulf of Aden on Monday, 177km north of Bosasso (Puntland) but escaped," he said. On Monday, the head of the group that seized the US ship Maersk Alabama vowed to retaliate for the deaths of three pirates in the military operation which rescued an American captain held on a lifeboat over the weekend. "The American liars have killed our friends after they agreed to free the hostage without ransom... this matter will lead to retaliation and we will hunt down particularly American citizens travelling our waters," Abdi Garad said by phone from the pirate lair of Eyl. "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." Meanwhile, the US takes "seriously" threats from pirates to avenge the deaths of fellow men killed by US Navy snipers, the Pentagon's top officer, Admiral Michael Mullen said Tuesday. "I certainly take their comments ... seriously," said Mullen, Chairman of the US joint chiefs of Staff, speaking to ABC television, after US Navy snipers on Sunday ended a five-day hostage drama by killing three pirates. Mullen said the US military is giving careful consideration to the risk of an increase in aggression during future pirate attacks in the perilous waters off Somalia. "We're very well prepared to deal with anything like that. And that would certainly be part of our review militarily," he said. Mullen added that review was part of an ongoing US military effort to confront the scourge of banditry on the open seas.