Undeterred Somali pirates went on a hijacking spree, brazenly capturing four more ships and taking over 60 crew members hostage in the Gulf of Aden, the waterway at the center of the world's fight against piracy. Pirates have vowed to retaliate for five colleagues slain by US and French forces in recent hostage rescues - and the top US military officer said Tuesday he takes those comments seriously. But Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, told ABC's “Good Morning America” that “we're very well prepared to deal with anything like that.” Despite Mullen's confident statement and President Barack Obama's warning of further US action, pirates captured two more nautical trophies Tuesday to match the two ships they seized a day or two earlier. The latest seizures were the Lebanese-owned cargo ship MV Sea Horse, the Greek-managed bulk carrier MV Irene E.M. and two Egyptian fishing boats. Maritime officials said the Irene carried 21 to 23 Filipino crew and Egyptian officials reported 36 fishermen, mostly Egyptians, on the two boats. It was not known exactly how many crew the Sea Horse had on board, but a ship that size would probably need at least a dozen. NATO spokeswoman Shona Lowe said pirates in three or four speedboats captured the Sea Horse on Tuesday - an attack that came only hours after the Irene was seized in a rare overnight raid. The Irene, flagged in the Caribbean island nation of St. Vincent and the Grenadines, was sailing from the Middle East to South Asia, Choong said. The Gulf of Aden, which links the Suez Canal and the Red Sea to the Indian Ocean, is one of the world's busiest and most vital shipping lanes, crossed by over 20,000 ships each year. US Navy Lt. Nathan Christensen, spokesman for the 5th Fleet, said the Irene carried 23 Filipino crew, while Choong reported it had 21 and Greek marine officials said it carried 22. There was no immediate way to reconcile the figures. Pirates in custody Three Somali pirates arrested during a French military rescue operation to free a group of hostages landed in France Tuesday and were taken into custody, prosecutors said. French special forces last week stormed The Tanit and freed four hostages including a three-year-old boy, days before a similar American operation to free a cargo ship's captain. However, the boy's father, 28-year-old yacht owner Florent Lemacon, was killed when commandos stormed the craft, six days after it was seized in one of the latest of a rash of pirate attacks off east Africa. Adventurer Lemacon's body is being brought back to France, with Pavy adding that an autopsy will be performed -- key to establishing if the adventurer was killed by a French bullet. Defence Minister Herve Morin has said he “cannot rule out” that the owner died from French gunfire.