Turkish officials say they have concrete evidence missing Saudi journalist Jamal Khashoggi was murdered, with a friend of the prominent writer saying they think he may have been dismembered.

Key points: A friend of Khashoggi says officials told him the journalist was dead

A friend of Khashoggi says officials told him the journalist was dead Khashoggi was a prominent critic of Prince Mohammed bin Salman

Khashoggi was a prominent critic of Prince Mohammed bin Salman The Washington Post said it would be a "monstrous and unfathomable act" if reports are true

A contributor to The Washington Post, Khashoggi has not been seen since Tuesday last week, when he entered the Saudi consulate in Istanbul, Turkey, to collect papers for his upcoming wedding.

Saudi officials said he left shortly afterwards but his fiancee, who was waiting outside, said he never came out.

Khashoggi, 59, who was once close to the Saudi royal family and has served as an adviser for senior Saudi officials, left the country last year to live in the US in self-imposed exile, saying he feared retribution for his criticism of Saudi policy in the Yemen war and its crackdown on dissent.

Khashoggi had been living in self-imposed exile in Washington. ( Reuters )

Turan Kislakci, a friend of Khashoggi and the head of the Turkish-Arab Media Association, told Associated Press that Turkish officials said the journalist has been brutally murdered.

"What was explained to us is this: 'He was killed, make your funeral preparations'," Mr Kislakci said.

"We called a few other places, these are lower officials, but they said: 'We have evidence he was killed in a barbaric way, we will announce it tomorrow or the day after'."

Mr Kislakci also alleged, based on conversations with officials he did not name, that Khashoggi was made to "faint", then was dismembered.

Turkish President Recep Tayyip Erdogan said he was closely following the case and that officials were examining camera footage and airport records as part of their investigation into the disappearance.

"Entries and exits into the embassy, airport transits and all camera records are being looked at and followed. We want to swiftly get results," he said, adding without explanation: "My expectation is still positive."

A security guard closes the entrance to a blocked road leading to the Saudi consulate in Istanbul. ( AP: Emrah Gurel )

But two Turkish sources told Reuters they believed Khashoggi was deliberately killed inside the consulate.

Saudi sources deny the accusations as baseless.

Last week, Prince Mohammed bin Salman said there was "nothing to hide", telling Bloomberg that Turkish authorities were welcome to search the consulate building and investigate.

In a statement, The Post said it would be a "monstrous and unfathomable act" if the reports of the murder are true.

"Jamal was — or, as we hope, is — a committed, courageous journalist," the director of The Post's editorial page said.

"He writes out of a sense of love for his country and deep faith in human dignity and freedom."

Who is Khashoggi?

Khashoggi had been critical of Saudi authorities. ( AP: Hasan Jamali, file photo )

A graduate of Indiana State University, Khashoggi began his career in the 1980s, covering the Soviet occupation of Afghanistan and the decade-long war that followed for the English-language daily Saudi Gazette.

He travelled extensively in the Middle East, covering Algeria's 1990s war against Islamic militants, and the Islamists' rise in Sudan, even interviewing Osama bin Laden in Afghanistan before al-Qaida was formed.

Khashoggi rubbed shoulders with the Saudi royal family and supported efforts to nudge the kingdom's entrenched ultra-conservative clerics to accept reforms.

He served as media adviser to Prince Turki al-Faisal, a former spy chief who was at the time the ambassador to the United States.

Prince Mohammed with Vladimir Putin at the Kremlin in 2017. ( Reuters: Pavel Golovkin )

Throughout his career he has been critical of authorities, frequently defending moderate Islamists and criticising Saudi foreign policy in the Middle East, particularly in Egypt and Syria.

In a September 23 interview with Turkey-based Syrian opposition television station just days before he disappeared, Khashoggi called Saudi Arabia's foreign policy "narrow minded", and ridiculed its crackdown on political Islam.

"This was a critical period in Arab history. I had to take a position. The Arab world had waited for this moment of freedom for a thousand years," Khashoggi said.

"The only recipe to get Iranians out of Syria — it is not Trump or anyone else — it is through the support of the Syrian revolution … Saudi Arabia must return to supporting the Syrian revolution and partnering with Turkey on this."

He disappeared just eight days later, with many pointing to his critique of Prince Mohammed and his policy as the reason behind his disappearance and alleged murder.

"As of now, I would say Mohammed bin Salman is acting like [Russian President Vladimir] Putin. He is imposing very selective justice," Khashoggi wrote in The Post last year after he fled the kingdom.

AP/Reuters