• Khashoggi visited Saudi consulate on Tuesday, then disappeared
• Security source says 15 Saudis flew to Istanbul on same day

Turkish authorities believe Saudi journalist Jamal Khashoggi was killed inside his country's consulate in Istanbul last week, Turkish sources said, in what they described as the deliberate targeting of a prominent critic of the Gulf kingdom's rulers.

Khashoggi, a former newspaper editor in Saudi Arabia and adviser to its former head of intelligence, left the country last year saying he feared retribution for his growing criticism of Saudi policy in the Yemen war and its crackdown on dissent.

On Tuesday he entered the Saudi consulate in Istanbul to get documents for his forthcoming marriage. Saudi officials say he left shortly afterwards but his fiancee, who was waiting outside, said he never came out.

"The initial assessment of the Turkish police is that Mr Khashoggi has been killed at the consulate of Saudi Arabia in Istanbul. We believe that the murder was premeditated and the body was subsequently moved out of the consulate," one of two Turkish officials told Reuters on Saturday.

An adviser to Turkey's President Tayyip Erdogan, Yasin Aktay, said authorities had concrete information about Khashoggi's case, and he believed the journalist was killed in the consulate.

A Saudi source at the consulate denied that Khashoggi had been killed at the mission and said in a statement that the accusations were baseless.

Another Turkish security source told Reuters that a group of 15 Saudi nationals, including some officials, had arrived in Istanbul in two planes and entered the consulate on the same day Khashoggi was there, and later left the country. The source said Turkish officials were trying to identify them.

Khashoggi is a familiar face on political talk shows on Arab satellite television networks and used to advise Prince Turki al-Faisal, former Saudi intelligence chief and ambassador to the United States and Britain.

Over the past year, he has written columns for newspapers including the Washington Post criticising Saudi policies towards Qatar and Canada, the war in Yemen and a crackdown on dissent which has seen dozens of people detained.