Turkey has ordered the formal arrest of nine executives and journalists from a leading opposition newspaper, broadcaster NTV reports.

The detention of the editor-in-chief and other senior staff from Cumhuriyet, a secularist title and one of the few media outlets still openly critical of President Recep Tayyip Erdoğan, has been slammed as "crossing a red line" against freedom of expression by the president of the European Parliament Martin Schulz.

Prosecutors in Istanbul said the paper's staff are suspected of committing crimes on behalf of Kurdish militants, linked to a failed coup attempt in July when rogue soldiers tried to seize power.

The country has led a tough crackdown since then, with more than 110,000 people sacked or suspended and 37,000 arrested. Some 170 newspapers, magazines, television stations and news agencies have also been shut down, leading to protests in support of journalists.

The operation has alarmed allies in the West and rights groups alike, who fear Erdoğan is using the coup to purge all dissent from Turkey - a NATO member which is applying to be a member of the European Union.

These fears were amplified on Friday, when Turkish authorities arrested the leaders of the main pro-Kurdish opposition party, sparking strong international condemnation.