The closed-door trial resumed Friday of two Turkish journalists accused of espionage and aiding a terrorist organization, a case that has heightened concerns over press freedoms in Turkey.

Cumhuriyet newspaper's chief editor Can Dundar and Ankara representative Erdem Gul face life imprisonment if found guilty of revealing state secrets over their report on alleged government arms-smuggling to Syrian rebels.

They published images that reportedly date back to January 2014, when local authorities searched Syria-bound trucks, leading to a standoff with Turkish intelligence officials. Cumhuriyet said the images proved Turkey was smuggling arms to Islamist rebels. The pair are accused of aiding the moderate Islamic movement led by U.S.-based cleric Fethullah Gulen, a foe of President Recep Tayyip Erdogan.

Human rights group say the two have done nothing but their job and the charges should be dropped. The case is seen as a bellwether of the future of press freedom in the country, which has witnessed a growing crackdown on independent and opposition media over the past few years.

"The ones who should be on trial are not us," Dundar said before the start of the second hearing.

Dozens of their supporters at the Istanbul courthouse where they are on trial chanted: "Free press cannot be silenced." Others came with their mouths taped over in an act of protest.

The journalists were arrested in November after Erdogan filed a personal complaint against the two. In February, Turkey's Constitutional Court ruled that their rights were violated and they were released from jail. Erdogan said he rejected the court's decision.

The Turkish president is facing increased criticism for his government's crackdown on free speech at home. Speaking in Washington, D.C., on Thursday, Erdogan insisted no journalist is in prison or on trial in his country because of their journalism work. He also said he welcomed criticism but would not tolerate insults.