Greek authorities have brought espionage charges against a British national working for a charity assisting migrants and a Greek soldier, who were arrested together in a military area on the border with Turkey.

Police had originally sought that the two men aged 25 and 29, who were arrested Sunday, should face lesser charges of entering a restricted military area without permission, according to the Associated Press (AP).

But on Monday, a prosecutor in the northeastern town of Orestiada charged them with espionage, because a photograph of military facilities was allegedly found in the men's possession, together with other pictures of the border area near the town of Nea Vissa in Evros province.

The 29-year-old Greek air force sergeant served in a border unit and was off-duty at the time of his arrest.

The Greek secret service also launched an investigation into the incident, Anadolu Agency (AA) said.

The Evros area is a key crossing-point for migrants entering Greece illegally from Turkey, and the possibility of the incident's links to a human smuggling scheme is also being investigated.