The pirates who hijacked an arms-laden Ukrainian tanker off the coast of Somalia have threatened to destroy the ship if no ransom is paid, a spokesman for the bandits said. The MV Faina is surrounded by US warships, and a Russian frigate is heading toward the scene, raising the stakes for a possible commando-style raid on the ship. "We held a consultative meeting for more than three hours today and decided to blow up the ship and its cargo - us included - if the ship owners did not meet our ransom demand," Sugule Ali told AP when a reporter called the ship via satellite telephone. "After three days, starting from tomorrow, the news of the ship will be closed. We know what to do next," he said. The pirates had said on Thursday they were willing to negotiate their ransom demand of $US20 million ($A29.3 million), after nearly two weeks of insisting they would never lower the price. 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. NATO ministers agreed on Thursday they would have seven ships in the area within two weeks. Six US warships already surround the Faina off the central coast of Somalia, and helicopters buzz overhead daily. Russia announced it would cooperate with the West in the fight, and several European countries have said they would launch an anti-piracy patrol.