Spokesman Commander John Harbour said the ships were hijacked around 2,000 kilometers off Somalia's coast.
Harbour said this attack is the furthest from shore to date.
Somali pirates have hijacked more than 20 ships since the beginning of March and are holding more than 300 crew members hostage.
In response to international naval forces patrolling the area, the pirates have extended their attacks deeper off the coast in the Indian Ocean.
Ships and crews are usually released unharmed after a ransom is paid.