Armed pirates hijacked a massive tanker as world powers on Saturday headed toward the Somali coast to end a two-week standoff aboard a ship laden with tanks and weapons, officials said.

The latest ship to be seized, a Greek vessel flying a Panamanian flag, was traveling from Southeast Asia to Europe, said Noel Choong, head of the International Maritime Bureau’s piracy reporting center in Kuala Lumpur. There were no further details on the ship, which was seized Friday.

Pirates who have been holding the Ukrainian MV Faina for two weeks have threatened to destroy the vessel unless a ransom is paid. Six U.S. warships are surrounding the Faina and a Russian frigate was heading toward the scene, raising the stakes for a possible commando-style raid on the ship.

Pirates have seized more than two dozen ships this year off the Horn of Africa, but the hijacking of the Faina has drawn the most international concern because of its dangerous cargo – 33 tanks and other heavy weapons.