Greek authorities are investigating a ship carrying 680 tonnes of explosives which was seized as it apparently sailed for Africa.
The ship's destination is still not known
Anti-terrorist forces and army bomb experts have begun examining the cargo of ammonia dynamite.
The ship's crew - said to be from Ukraine and Azerbaijan - are expected to be questioned later on Monday by public prosecutors.
The ship, the Baltic Sky, was boarded by Greek commandos late on Sunday following a tip-off - possibly from an international intelligence agency - that it had suspicious cargo on board.
A Greek Government spokesman said the Baltic Sky seemed to be heading for Africa, although he did not mention a particular destination.
Other reports said that the ship had loaded its cargo in a north African port before sailing towards a destination in east Africa.
But that route would not take the ship along the western Greek coast.
The vessel is owned by a company registered in the Marshall Islands in the Pacific, but is registered in the Comoro Islands, which is regarded as a provider of flags of convenience, the merchant marine ministry announced.
The ship looks old and the crew members were asking for supplies... maybe it had lost its route
"It looks certainly like a suspect ship," government spokesman Telemachos Hytiris said.
"This is a huge amount of explosives and a great success for
However the BBC's Panos Polyzoidis in Greece says the shipment of explosives is so large that it may have been intended for a government rather than an organisation, and may be for use by the mining industry.
One report said that the load had been bound for Tunisia, which has a large mining industry, but this could not be confirmed.
And the mayor of Astakos, the main town near the port where the ship is being docked, said the ship and crew had not appeared overly suspicious.
"The ship looks old and the crew members were asking for supplies," he told Reuters news agency.
"Maybe it had lost its route."
Local officials have also expressed safety concerns despite the ship being heavily guarded, and have asked that the ship be moved, as detonators and fuses are also on board along with the explosive cargo.
Nato forces have been inspecting ships in the region for some time.
However patrols have been stepped up in the eastern Mediterranean in recent months in the aftermath of the recent bombing attacks in Saudi Arabia and Morocco, in which dozens were killed.
Greece has also stepped up its border security as it prepares for the 2004 Olympics and in a bid to stem the flow of illegal immigrants attempting to enter the country.