Khashoggi, a Washington Post columnist and a critic of Saudi Crown Prince Mohammed bin Salman, disappeared after entering the consulate to obtain paperwork necessary for his upcoming marriage to Cengiz, a Turkish national.
Erdoğan reiterated during an address to parliament on Friday that Riyadh must disclose the location of Khashoggi's body and identify who ordered his killing - a sign that Ankara is willing to keep up the pressure on the beleaguered kingdom and its de facto ruler, the crown prince Mohammed bin Salman.
Turkish police have been investigating the case, and Turkish President Recep Tayyip Erdoğan has said 18 people arrested in Saudi Arabia over the killing should be sent to Turkey to face the trial.
"Information from the Turkish side affirms that the suspects in Khashoggi's case premeditated their crime", said a statement from the Saudi public prosecutor.
Trump has repeatedly said that he wants to get to the bottom of what happened but doesn't want to jeopardise the United States's relationship with Saudi Arabia, and in particular, the billions of dollars in arms purchases that the Saudi armed forces make each year from American defense contractors.
Media captionJamal Khashoggi: What we know about the journalist's disappearance and death What else did his fiancée say?
Cengiz, who is studying for a PhD in Istanbul, also said she declined an invitation to the USA from President Donald Trump, a staunch defender of the Gulf kingdom.
She also said she had declined an invitation to meet Donald Trump, saying the US President was not honest about investigating the killing.
On the sidelines of a Syria summit in Istanbul, the two leaders had a "relaxed exchange", the Elysee palace said, and agreed not to announce their next positions on the issue without first coordinating "at the European level".
"With our collective interests in peace and unwavering respect for human rights in mind, the murder of Jamal Khashoggi in a diplomatic facility must concern us all greatly", he said.
A demonstrator holds picture of Jamal Khashoggi during a protest in front of Saudi Arabia's consulate in Istanbul in Turkey.
On Thursday, CIA Director Gina Haspel briefed Trump about her recent trip to Turkey, where she listened to purported audio of Khashoggi's killing. In the call, Merkel condemned Khashoggi's killing at the kingdom's consulate in Istanbul "in the sharpest possible manner".
Mr Trump also said Riyadh had staged the "worst cover-up ever" over the killing.
But she said that Khashoggi considered Turkey to be a safe country, and believed that if he was detained or interrogated, the situation would be quickly resolved.