MOGADISHU, Somalia - A U.S. destroyer off the coast of Somalia closed in Saturday on a hijacked Ukrainian ship loaded with tanks and ammunition, watching it to ensure the pirates who seized it do not try to remove any cargo or crew.

As Russian and American ships pursued the hijackers of the Ukrainian vessel, pirates seized another ship off Somalia's coast, an international anti-piracy group said.

The Greek tanker with a crew of 19 is carrying refined petroleum from Europe to the Middle East. It was ambushed Friday in the Gulf of Aden, said Noel Choong, who heads the International Maritime Bureau's piracy reporting centre based in Malaysia. He said pirates chased and fired at the ship before boarding it.

In Somalia, a man claiming to be spokesman of the pirates holding the Ukrainian ship said the hijackers want $35 million to release the vessel. But there was no way to immediately verify his claim that he represented the pirates.

On Thursday, pirates seized the Ukrainian-flagged ship Faina en route to Kenya with 33 Russian-built T-72 tanks and a substantial quantity of ammunition and spare parts. Russia's navy said Friday it had dispatched a warship to the area, and the United States said American naval ships were tracking it with special concern because of the weaponry on board.

The hijackings were the latest in a series of audacious maritime attacks off the coast of Somalia, a war-torn country that has been without a functioning government since 1991.

A U.S. defence official said the destroyer USS Howard is pursuing the hijacked Ukrainian vessel and is now within a few thousand metres of it. The hijacked ship is anchored a few kilometres off the Somalia coast, said the official who spoke on condition of anonymity due to the sensitive nature of the situation.

The destroyer is watching to make sure the pirates do not try to remove anything, the official added.

The USS Howard's website says it is equipped for combat operations at sea with surface-to-air missiles, Tomahawk cruise missiles, antisubmarine rockets, torpedoes, and a rapid-fire deck gun.

Kenyan government spokesman Alfred Mutua said the Faina had not yet docked at any port and was still at sea.

Kenya "is not aware of any credible (ransom) demand being made," Mutua said in statement. He said Kenya "does not and will not negotiate with international criminals, pirates and terrorists."

Ukraine's Foreign Ministry said the Kenyan Defence Department was using its contacts to try to resolve the problem. It said Kenyan authorities were sharing information with Somalia, Ukraine, Russia, the U.S. and Britain in an effort to secure the swift release of the ship and its crew.