A group of writers, including Nobel laureates, are calling on Turkey to drop charges against two prominent journalists who face life imprisonment for their reports, and to end its crackdown on free expression.

In an open letter addressed to Prime Minister Ahmet Davutoglu, PEN International on Thursday condemned what it said was an "increasing climate of fear, censorship and the stifling of critical voices."

The two journalists, Can Dundar and Erdem Gul, go on trial on Friday accused of espionage and other charges for their reports on alleged government arms smuggling to Syrian rebels. The two were jailed for three months until a court ruled for their release in February.

The writers said the two are facing prison "simply for carrying out their legitimate work as journalists."