Maritime authorities in Greece say a ship carrying nearly 700 metric tons of explosives, which they seized on Sunday, has not been legally registered.
The ship sailed around the Mediterranean for six weeks
The Baltic Sky had been sailing under a Union of Comoros flag but its registration was revoked on 3 June for numerous irregularities.
Under international maritime law, any ship without a flag can be detained as soon as it enters port.
It is a ghost ship that would have been seized anywhere it went, whether it was carrying explosives or potatoes
Greek merchant marine minister
The seizure, which took place in Greek territorial waters, has sparked a diplomatic row involving Sudan and Tunisia, the stated recipient and point of loading, who say the cargo was for civilian purposes.
The five Ukrainian and two Azerbaijani crew members have been charged with illegal possession and transport of explosives.
"The ship is not legally registered, it does not belong to the flag which it flies," Merchant Marine Minister George Anomeritis said.
"It is a ghost ship that would have been seized anywhere it went, whether it was carrying explosives or potatoes."
THE BALTIC SKY'S COURSE
13 May: Left Tunisian port of Gabes for Port Sudan
21 May: Passes through Dardanelles Straits
2 June: Docks in Istanbul
5 June: Leaves for Suez
6-22 June: Zigzags around the Aegean Sea
22 June: Boarded by Greek forces in southern Ionian Sea
The ship had apparently been sailing around the Mediterranean for six weeks before being impounded, instead of heading straight for Khartoum.
It was boarded in the Ionian Sea following a tip-off and then forced into the tiny Greek port of Platiyali, 235 kilometres (145 miles) north-west of Athens.
The discovery of the cargo comes amid heightened terror alerts in East Africa and elsewhere.
But Greek investigators believe that the vessel's owners may have been simply delaying the cargo's delivery so that they could charge more money for it.
All efforts to trace the owners, who are believed to be based in Ireland, have failed.