- Video report by ITV News International Affairs Editor Rageh Omaar
Turkish President Recep Tayyip Erdogan has called on Saudi Arabia to prove its claim that a missing Saudi journalist left the kingdom's consulate in Istanbul alive.
Jamal Khashoggi, a high profile critic of the Saudi regime, went missing after visiting the consulate on 2 October to complete paperwork needed to marry his Turkish fiancee. He has not be seen since.
Turkish officials say the 59-year-old Washington Post contributor was killed at the consulate and that his body was later removed from the building.
Saudi officials deny the allegations, with the consulate adamant Khashoggi left its premises.
The United States and other Saudi allies have taken a cautious approach, expressing concern but refraining from any strong comments against the oil-rich kingdom.
President Donald Trump told reporters he was "concerned".
"Hopefully that will sort itself out. Right now, nobody knows anything about it," he said.
Speaking during a visit to Hungary, Erdogan said the Saudis "cannot save themselves by saying he left. They have to provide proof."
Erdogan said Turkish police, intelligence and judiciary officials are investigating Khashoggi's disappearance.
Earlier, a Saudi ambassador to Turkey was summoned to the ministry to request Riyadh's cooperation in the investigation.
The United Nations said Secretary-General Antonio Guterres is "very concerned" about Khashoggi's disappearance and is reportedly watching the investigation "closely."
Khashoggi's colleagues at the Washington Post said it would be a "heinous crime" if Turkey's claims of a state-sponsored murder were true.
Global Opinions writer Jason Rezaian said the columnist had often expressed concern about his safety due to his outspoken opinion pieces that not only criticised the regime, but the Crown Prince Mohammed bin Salman who is keen to maintain close links with the West.
Khashoggi left Saudi Arabia amid a crackdown on intellectuals and activists who criticised the policies of Prince Mohammed, the 33-year-old heir to the throne now held by his 82-year-old father, King Salman.
Although there have been some reforms, such as allowing women to start driving cars, Prince Mohammed's reign has seen businessmen, royals, activists and others rounded up and arrested.
Azzam Tamini, a friend of Khashoggi, told ITV News that sources very close to the leadership in Turkey had confirmed to him that the writer was dead and that they had documentation to prove it.