The higher Istanbul court rejected the appeal from lawyers for the Cumhuriyet daily's editor-in-chief Can Dundar and its Ankara bureau chief Erdem Gul who had been arrested on "spying" charges last week, the Dogan news agency reported.
The case of the two journalists -- both hugely prominent figures on the Turkish media scene -- has amplified concerns about press freedom in Turkey under the rule of President Recep Tayyip Erdogan.
Protesters demonstrate against arrest of Turkish opposition journalists
Media rights watchdog Reporters Without Borders (RSF) Tuesday launched an international petition for the release of the pair, with initial signatories including US linguist Noam Chomsky, French economist Thomas Piketty and Turkish pianist Fazil Say.
The petition accuses Erdogan of "waging a methodical crackdown on the media in Turkey for years" and persecuting journalists "in an increasingly ferocious manner".
"The Erdogan regime's arrests, threats and intimidation are unworthy of a democracy," added the petition.
RSF secretary general Christophe Deloire said the Turkish authorities appeared to prosecute journalists more than IS militants. "The Turkish authorities are targeting the wrong enemy," he said at a news conference in Istanbul.
Turkish police raid media group critical of Erdogan
An Istanbul court on Thursday charged the two journalists with "aiding a terrorist organisation" and spying for alleging that Turkey had covertly shipped arms to rebels in Syria.
Both were remanded in custody pending trial, but no date has been set and it may be months away. If convicted, both men face up to 45 years in prison.
Erdogan had previously warned Dundar he would "pay a heavy price" for the story.
The case concerns the January 2014 interception of a convoy of trucks near the Syrian border. Cumhuriyet published images and video claiming to show they were carrying boxes of weapons and ammunition bound for rebels in Syria sent by Turkey's spy agency.
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