Armed pirates hijacked a Greek bulk carrier with 25 crew members off the coast of Somalia on Thursday, the 13th ship seized in the notorious African waters in the past two months, a maritime official said. The ship was en route to Kenya when it was attacked off Somalia's eastern coast, said Noel Choong of the International Maritime Bureau. He said the latest incident showed that Somali pirates, who were previously operating off the country's northern coast in the Gulf of Aden, have to begun to attack ships in the east. A multinational naval force patrolling the area has been informed, and ships have been warned to stay clear of Somalia's coast, he said. "We advise ships to stay at least 250 miles (400 kilometers) from the coast and even then, they must maintain a strict watch,'' Choong said.