MOSCOW, October 23 (RIA Novosti) - Pirates, who have been holding a Greek-owned cargo ship since May 2, pledged to start killing Ukrainian crew members as soon as fuel on the ship runs out, the Russian Seamen's Trade Union said on Friday.

The union urged Ukrainian President Viktor Yushchenko to take immediate action to release 24 Ukrainian sailors.

"All crew members need aid, the situation on the vessel is critical: pirates threaten to kill everybody as soon as fuel runs out and the diesel generator stops," the trade union said on its website.

Water, food and medicines at the ship have run out completely, the union reported.

The Ariana cargo ship was seized by pirates 250 miles southwest of the Seychelles in the Indian Ocean. The ship was en route to the Middle East from Brazil and was carrying 35,000 metric tons of soya. The pirates, who demand a ransom of $10 million, escorted the ship to the Somali port of Hobyo.