Turkish prosecutors have demanded life sentences for two top journalists from the opposition Cumhuriyet newspaper on charges of revealing state secrets, a report says.

The state secrets in question refer to an investigative report by Cumhuriyet that President Recep Tayyip Erdogan's government tried to ship arms to Islamists in Syria.

Prosecutors asked the Istanbul court to sentence editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul each to one aggravated life sentence, one ordinary life sentence and 30 years in jail, local media reported quoting the indictment.

The Cumhuriyet report said both Mr Erdogan and his hugely powerful but low-profile ally, the head of the National Intelligence Organisation Hakan Fidan, are named as plaintiffs in the indictment.

Turkish journalists hold banners and shout slogans during a demonstration in support of the charged journalists. ( AFP: Adem Altan )

Mr Dundar and Mr Gul were both placed under arrest in late November over the report earlier in the year that claimed to show proof a consignment of weapons seized at the border in January 2014 was bound for Islamist rebels in Syria.

Since then, they have both been held in the Silivri jail on the outskirts of Istanbul ahead of their trial, whose date has still yet to be announced.

In the indictment, they have been formally charged with obtaining and revealing state secrets "for espionage purposes" and seeking to "violently" overthrow the Turkish government as well as aiding an "armed terrorist organisation", it said.

The penalties demanded by the prosecutors are significantly higher than had previously been expected.

The case has amplified concerns about press freedom under the rule of Mr Erdogan, who had personally warned Mr Dundar he would "pay a price" over the front-page story.

AFP