Photo released by the U.S. Navy shows Somali pirates in small boats alongside the hijacked 'Faina', 28 Sep 2008

Somali pirates who seized a Ukrainian ship transporting weapons to east Africa say the vessel has been surrounded by three foreign warships.

A man who identified himself as pirate spokesman Sugule Ali told reporters Sunday that the pirates will not surrender, despite being under siege by a U.S. warship and two European warships.

Speaking by satellite telephone, Ali said the pirates are demanding a $20-million ransom for the release of the vessel. Reports on Saturday said the rebels wanted $35 million.

The U.S. Navy said Sunday that a destroyer is tracking the Ukrainian ship, the MV Faina, in the Gulf of Aden. The nationalities of the other warships are not known.

Twenty-one people are aboard the ship, which was seized on Thursday as it traveled to Kenya. The pirates say another member of the ship's crew died of natural causes.

The hijacked ship is carrying 33 Russian-made T-72 tanks, as well as ammunition and spare parts for the armored vehicles.

A Canadian Navy warship Ville de Quebec, foreground, secures safe sailing as they escort a World Food Programme ship on off the coast of Somalia (File)

A Kenyan maritime official with the East Africa Seafarers' Association said the weapons appear to belong to South Sudan, which is barred from arms sales under a 2005 U.N.-brokered peace deal.

But Kenyan government spokesman Alfred Mutua said pirates are spreading what he called "alarming propaganda" that the seized weapons do not belong to Kenya's armed forces.

On Friday, the Russian Navy said it was sending a frigate to Somalia's coast.

In other developments, maritime officials say Somali pirates seized a Greek tanker and its crew Friday in the Gulf of Aden. The 19 crew members are Romanians.

Authorities announced Saturday that Somali pirates had released vessels from Egypt, Malaysia and Japan along with their crews.

Some information for this report was provided by AFP and AP.