Greek police announced on Saturday that the country's authorities intend to file espionage charges against two citizens.
The Greek men were arrested for allegedly supplying naval information to Turkey from the island of Rhodes
One of the suspects was working aboard a ferry from Rhodes to the Greek island of Kastellorizo, just two kilometres off the Turkish coast.
The other was said to be working at the Turkish consulate in Rhodes.
Kastellorizo is in disputed waters that have been a source of escalating tensions between Turkey and Greece.
Greek police said the investigation on which charges will be brought "was conducted in close co-operation with the Greek National Intelligence Service".
The men are accused of photographing and recording warships and military installations, and passing on the information to Turkey.
Tension between Turkey and Greece increased over summer when Ankara sent a survey vessel to map out energy drilling prospects in waters also claimed by Athens.
Last week, Greece said it would not begin formal talks with Turkey over the maritime row while Turkish vessels remained in the contested waters.
Greece, France and Cyprus had pushed for tough action against Turkey.
But other EU nations including Germany, Italy and Poland opposed sanctions or an embargo on a fellow Nato member.
There has been increasing concern that Turkish intelligence agencies have increased operations in mainland Europe.
Recent allegations have focused on Turkish efforts to spy on political opposition groups.
In 2017, the German government announced it was investigating "intolerable" activities by suspected Turkish agents against political dissidents.
Those accusations re-emerged in May this year, when the German government claimed Turkey was also spying on MPs.