A Russian crew member of a hijacked Ukrainian-operated cargo ship, boarded by pirates off the coast of Somalia three days ago, died of hypertension on Sunday, the captain of the vessel said.

Viktor Nikolsky reported the death to a U.S. news agency from the deck of the MV Faina via a satellite phone, provided by one of the pirates who seized the ship last Thursday.

Nikolsky also told the Associated Press he could see a U.S. ship about one kilometre from his freighter.

The U.S. navy destroyer, USS Howard, is stationed off the coast of Somalia. According to an unidentified U.S. official quoted by AP, the destroyer's crew is watching for any movement of cargo off the ship.

The Faina was headed to Kenya with 21 crew members and a load of ammunition, 33 Russian-built T-72 tanks and spare parts — ordered by the Kenyan government — when pirates seized the ship.

At the time, 17 Ukrainians were reported on board, along with three Russians and one Latvian.

A man who identified himself as a spokesman of the pirates, Sugule Ali, said via satellite phone on Sunday that hijackers want $20 million US to release the vessel.

With files from the Associated Press