Turkish President Recep Tayyip Erdogan has challenged Saudi Arabia to provide proof that a missing journalist has left its consulate in Istanbul.
Saudi national Jamal Khashoggi was last seen visiting the building last week. US President Donald Trump expressed concern about his fate on Monday.
Turkey requested a search of the consulate, after saying that Mr Khashoggi had been murdered within its walls, media reports say.
Saudi Arabia has denied the claims.
"Consulate officials cannot save themselves by saying that he left the building," Mr Erdogan told a news conference on Monday.
"If he left, you have to prove it with footage."
On Sunday Turkish officials said investigators had "concrete proof" of the killing which, they said, was carried out by a 15-person Saudi team who arrived in the country last week.
No evidence has been presented. Crown Prince Mohammed bin Salman has previously said officials are welcome to conduct a search as there is nothing to hide.
Mr Khashoggi had been living in the US, where he contributed articles to the Washington Post's opinion section. The newspaper said the US should demand answers from the Saudis.
Mr Trump later told reporters at the White House: "I am concerned about it. I don't like hearing about it.
"Hopefully that will sort itself out. Right now nobody knows anything about it."
Meanwhile, US Secretary of State Mike Pompeo said: "We call on the government of Saudi Arabia to support a thorough investigation of Mr Khashoggi's disappearance and to be transparent about the results of that investigation."
When was he last seen?
Jamal Khashoggi went to the consulate last Tuesday to obtain a document certifying he had divorced his ex-wife, so that he could remarry his Turkish fiancée, Hatice Cengiz.
She said Mr Khashoggi had been required to surrender his mobile phone, which is standard practice in some diplomatic missions.
He told her to call an adviser to President Erdogan if he did not return.
What has Turkey said?
Turkish officials say Mr Khashoggi was killed on the premises and his body was then removed.
The head of the Turkish-Arab Media Association, Turan Kislakci, told the New York Times that Turkish police officers providing security for the consulate had checked their security cameras and did not see the journalist leave on foot.
But he added that diplomatic cars had been seen moving in and out.
Mr Erdogan was more circumspect, saying on Sunday he remained "positive" and would await the results of an investigation.
Turkey's NTV broadcaster reported that a request had been made to search the consulate after the meeting with the ambassador.
We may never know
Analysis by BBC Security Correspondent Frank Gardner
The allegations of what the Saudi authorities may have done to one of their most prominent critics are - if proven - horrific. But the evidence has yet to be produced, leaving everyone guessing what could have happened to him after he entered the Saudi consulate in Istanbul.
I have known Jamal Khashoggi for 18 years and although outspoken, he was not one to take unplanned risks. Yes, he knew he had riled the all-powerful Crown Prince Mohammed Bin Salman with his widely read criticisms of MBS' increasingly heavy-handed rule. But Mr Khashoggi thought he had the situation under control.
Instead, Saudis I have spoken to believe that, despite official denials, one possible scenario is that he was abducted inside the consulate, driven out in a diplomatic vehicle and "rendered" back to Saudi Arabia to either face retribution or be held incommunicado under indefinite house arrest.
The worrying thing is, we may never know.
What have the Saudis said?
Saudi Arabia said the allegations were baseless. It has allowed journalists into the consulate to show Mr Khashoggi is not there, reportedly even opening cupboards.
On Wednesday, Crown Prince Mohammed told Bloomberg News that his government was "very keen to know what happened to him", and that Mr Khashoggi had left "after a few minutes or one hour".
Who is Jamal Khashoggi?
He is a high-profile critic of the crown prince. Mr Khashoggi, 59, has more than 1.6 million Twitter followers and has written for the Washington Post opinion section.
A former editor of the al-Watan newspaper and a short-lived Saudi TV news channel, Mr Khashoggi was for years seen as close to the Saudi royal family. He served as an adviser to senior Saudi officials.
After several of his friends were arrested, his column was cancelled by the al-Hayat newspaper and he was allegedly warned to stop tweeting, Mr Khashoggi left Saudi Arabia for the US.
There he wrote opinion pieces for the Washington Post and continued to appear on Arab and Western TV channels.
How does this play out politically?
The allegation alone is enough to damage the countries' relationship, which has already been under some strain.
Turkey backed Qatar when it was being targeted by a Saudi blockade. Turkey also backed Saudi Arabia's arch-rival Iran during its US trade disputes.
Mr Khashoggi's disappearance also puts pressure on President Trump to get tougher on Saudi Arabia, a strong Middle East ally.