A court in Istanbul on Thursday charged two journalists from the opposition Cumhuriyet newspaper with spying after they alleged Turkey's secret services had sent arms to Islamist rebels in Syria, Turkish media reported.
Editor-in-chief Can Dundar and Erdem Gul, the paper's Ankara bureau chief, are accused of spying and "divulging state secrets", the reports said. Both men were placed in pre-trial detention.
According to Cumhuriyet, Turkish security forces in January 2014 intercepted a convoy of trucks near the Syrian border and discovered boxes of what the daily described as weapons and ammunition to be sent to rebels fighting against Syrian President Bashar al-Assad.
It linked the seized trucks to the Turkish National Intelligence Organisation (MIT).
The revelations, published in May, caused a political storm in Turkey, with an enraged President Recep Tayyip Erdogan vowing Dundar would pay a "heavy price".
He personally filed a criminal complaint against Dundar, 54, demanding he serve multiple life sentences.
Turkey has vehemently denied aiding Islamist rebels in Syria, such as the Islamic State group, although it wants to see Assad toppled.
"Don't worry, this ruling is nothing but a badge of honour to us," Dundar told reporters and civil society representatives at the court before he was taken into custody.
Reporters Without Borders had earlier Thursday urged the judge hearing the case to dismiss the charges against the pair, condemning the trial as "political persecution".
The Cumhuriyet daily was awarded the media watchdog's 2015 Press Freedom Prize just last week, with Dundar travelling to Strasbourg to receive the award.
"If these two journalists are imprisoned, it will be additional evidence that the Turkish authorities are ready to use methods worthy of a bygone age in order to suppress independent journalism in Turkey," said RSF secretary general Christophe Deloire in a statement.
Reporters Without Borders ranked Turkey 149th out of 180 in its 2015 press freedom index last month, warning of a "dangerous surge in censorship".