A Tunisian munitions company meanwhile said it had a contract to supply the explosives to a Sudanese firm for civilian use.

But the Societe Tunisienne d'Explosifs et Munitions (SOTEMU) said the vessel had changed its route and had gone instead to the Black Sea, and the ship's owner had then threatened to confiscate the cargo and re-sell it.

The Baltic Sky was carrying 680 tonnes of explosives, mainly TNT -- the "equivalent of an atomic bomb" -- and 8,000 detonators when Greek special forces seized it on Sunday, Greek Merchant Marine Minister George Anomeritis told a press conference.

The ship's crew -- five Ukrainians and two Azerbaijanis -- were arrested and charged with possessing and transporting explosives to third parties for outlawed activities, the semi-official Athens News Agency (ANA) reported.

Greek prosecutors also filed charges against the ship's operator Alpha Shipping, registered in the Marshall Islands in the south Pacific.

The seizure of the ship came amid a months-long heightened state of security in Greece, which currently holds the EU's rotating presidency and whose capital Athens is due to host the Olympic Games in 2004.

Acting on a tipoff received five days ago, Greek forces seized the Baltic Sky, flying the flag of the Comoros Islands, while it was sailing through the Ionian Sea off the west coast of Greece.

One ministry official said the vessel was "packed to breaking point".

"This is the biggest quantity of explosives ever seized in the world from a boat sailing illegally," the merchant marine ministry said in a statement.

Ship documents showed the explosives were destined for a company called Integrated Chemicals and Development, whose only address was listed as a post office box in Sudan's capital of Khartoum, Anomeritis said.

But an investigation so far showed that no such company existed, he said.

Sudan, in eastern Africa, is on a US list of nations that support terrorism, but Anomeritis refused to speculate whether the explosives were destined for terrorist-related activities.

"As no one knows where the explosives were going, no one knows what they were going to be used for," he insisted. "The probe is still underway."

The Tunisian firm SOTEMU said it had a contract to supply the explosives to Integrated Chemicals and Development.

It said Sudanese authorities issued an import license, and the Baltic Sky was loaded on May 12 and 13 in the Tunisian port of Gabes, from where it left bound for Port Sudan on May 13.

But it said the vessel, chartered from a Cypriot company, Aegeas Navigation Ltd, went instead to the Black Sea, which it left on June 2, and SOTEMU alerted Tunisian authorities.

It said the ship's Istanbul-based owner then threatened to confiscate the cargo and re-sell it if the company did not pay an additional sum, forcing SOTEMU to refer the matter to the public prosecutor in Tunisia.

SOTEMU said the cargo comprised "industrial explosives frequently used in mines and quarries, as well as in oil exploitations."

The Greek ministry said the authenticity of the Baltic Sky's shipping documents were still being checked but they corresponded exactly to the cargo.

According to the ship's log, the vessel sailed around the Mediterranean Sea during May and June.

It left Albania on April 27 and docked in Gabes on May 12. It then passed through Turkey's Dardanelles Strait to arrive in the Turkish city of Istanbul on May 22. It travelled through the strait once again on June 2.

"It appears that at some stage someone realised that they were carrying a cargo that wasn't destined for anyone and maybe they were trying to get rid of it somewhere," the ministry said in a bid to explain the ship's erratic voyage.

WAR.WIRE