The cargo ship, Nour M, was taken to the island of Rhodes where its Turkish captain and seven crew members were placed under arrest, coastguard sources said.
In a statement, the coastguard said efforts to give a full account of the firearms and ammunition on board the cargo ship were ongoing.
"The exact destination of the arms and ammunition has yet to be verified," the coastguard statement said.
The state-run Athens News Agency reported that the vessel, which was intercepted near the Greek island of Symi, had set sail from Ukraine and was destined for the Turkish port of İskenderun.
Aside from the firearms, it was also allegedly carrying a large quantity of explosives, ANA said.
The agency noted that the ports of Tartus in Syria and Tripoli in Libya had also been declared as destination ports to marine traffic systems, while İskenderun was declared as the destination port by the ship's captain.
The Nour M is also believed to have been used in the past for drug trafficking, ANA said.