Istanbul (AFP) - Turkish prosecutors on Wednesday sought multiple life sentences for two top opposition journalists on charges of revealing state secrets in a report that alleged President Recep Tayyip Erdogan's government tried to send arms into Syria.

Prosecutors asked the Istanbul court to sentence Cumhuriyet newspaper editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul each to a penalty comprising one aggravated life sentence, one ordinary life sentence and 30 years in jail, the Dogan news agency reported.

The severity of the demand intensified concerns about press freedom in Turkey, with EU Enlargement Commissioner Johannes Hahn saying he was "shocked".

It is not unusual in Turkey for prosecutors to seek such combined sentences, but this is done in cases involving violent crimes such as murder.

An aggravated life sentence means tougher conditions, including restricting a prisoner's leisure hours.

Both Erdogan and the head of the National Intelligence Organisation (MIT) Hakan Fidan -- the president's hugely powerful but low-profile ally -- are named as plaintiffs in the 473-page indictment, Dogan said.

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

The pair have been held in Silivri jail on the outskirts of Istanbul ahead of their trial, whose date has yet to be announced.

They have been 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", according to the indictment.

The penalties demanded by the prosecutors are far heavier than expected.

- 'Shocked by demand' -

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

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"Shocked by life sentences demanded... Equality (and) proportionality before the law are a must," Hahn, who visited Turkey on Monday, said in a tweet.

He warned Turkey that as it negotiates EU membership it must ensure "full respect" of human rights.

US Vice President Joe Biden, on a visit to Istanbul last week, complained that media were being "intimidated or imprisoned for critical reporting" in Turkey.

"That's not the kind of example that needs to be set," said Biden, who also met with Dundar's wife and son in talks that hugely irritated the Turkish government.

- 'You can't defeat us' -

News of the indictment coincided with the publication in Istanbul by Human Rights Watch of its annual report, which denounced the prosecution of the pair.

"We are absolutely clear that Can Dundar and Erdem Gul, in publishing stories on that subject were doing their job as journalists and nothing more than that," HRW's Turkey representative Emma Sinclair Webb told reporters in Istanbul.

"Turkish political leaders, especially the president, have showed an unprecedented willingness over the last years to create a climate of fear for their critics and demonise their opponents."

Delegates from press freedom groups like the Committee to Protect Journalists and Reporters Without Borders (RSF) joined a vigil outside the jail to support the reporters, urging Turkey to free them "without delay".

In a statement, RSF accused the prosecutors who issued the sentencing demand of "culpable cruelty".

Dundar, now detained for a 63rd day according to Cumhuriyet, has not stopped writing columns for his paper and in his latest article attacked Erdogan for seeking to suppress dissent.

"They are trying to limit us, this society, this country and this world to one colour," he wrote.

"Only the 'chief' would be allowed to speak, everyone would praise him and not a single objection would be made."

But he added: "Even if you make us pay the heaviest price, we will continue to tell and write the truth. You cannot defeat us."