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 Georges 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 in Africa, 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 Afica, 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 authenticity of the Baltic Sky's shipping documents were still being checked but they corresponded exactly to the cargo, the Greek ministry said.

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 the Tunisian port of 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 a explain the ship's erratic voyage.

The ship's Ukrainian captain, 64-year-old Anatoly Baltak, did not tell the Greek coast guard of his volatile cargo when officers boarded the Baltic Sky.

Monday's charges against the crew also include a misdemeanor of violating international sea law by failing to notify Greek authorities that the ship carried dangerous materials 24 hours before entering the country's territorial waters, ANA said.

After it was seized, special Greek patrol boats escorted the ship into the tiny merchant port of Platy-Yali, on Greece's western coast.

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