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 man places a poster of missing Saudi writer Jamal Khashoggi, on a barrier that blocks the road leading to the Saudi Arabia consulate in Istanbul, as people gather in his support, Friday, Oct. 5, 2018.
The ambassador declined to discuss the matter further, instead saying, "We don't want to harm the investigation". "The consulate officials can not save themselves by simply saying "he has left", Erdogan told a news conference in Budapest, Hungary, where he is on a two-day visit.
Khashoggi, a columnist for The Washington Post who had been living in self-imposed exile in the United States, entered the Saudi Consulate in Turkey seeking a document he needed to get married and has not been seen since.
"If a person walks into premises and if he isn't stepping out, then who has to prove it?"
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 U.S., from where he wrote opinion pieces for the Washington Post and continued to appear on Arab and Western TV channels.
A video still from nearby closed circuit television cameras released overnight by the police showed Mr Khashoggi entering the building. The private DHA agency said the planes, which it identified as a two Gulfstream jets belonging to a Riyadh-based company that hires private aircraft, landed at Istanbul's Ataturk Airport on the day Khashoggi vanished.
Turkey has asked for permission to search Saudi Arabia's consulate for prominent Saudi journalist Jamal Khashoggi, who went missing last week, broadcaster NTV said on Monday.
The Washington Post has published an image it described as the last surveillance photograph of its missing Saudi contributor.
"I know many in Washington, and the world over share this concern for his wellbeing". RSF wrote that a series of cables between Saudi embassies and the Saudi foreign ministry (made public by WikiLeaks) revealed that "extraordinary initiatives" were considered by Riyadh in an attempt to rehabilitate its worldwide image. "It has been conveyed to him that we expect full coordination in the investigation process", the official said. But if Khashoggi is dead, the Saudi government certainly had a motive to kill him.
Still, killing someone like Khashoggi, who long had ties to the royal family and the Saudi intelligence apparatus, in a consulate would be a major escalation in the prince's rise.
Speaking for the first time about the allegations, a UK Foreign Office spokesman said: "These are extremely serious allegations".
"We are aware of the latest reports and are working urgently to establish the facts, including with the government of Saudi Arabia", the representative said.
The Saudi source said that a security team including Saudi investigators had arrived in Istanbul on Saturday to take part in the investigation into Khashoggi's disappearance.
He reportedly went to the consulate in Istanbul for paperwork to marry his Turkish fiancée. On the eve of his planned marriage to a Turkish woman, he entered the consulate on 2 October and has not been seen since.
In a statement carried by Saudi Press Agency, the official strongly denounced these baseless allegations.
On Sunday, Turkish President Erdogan said that it was very saddening that Khashoggi disappeared in Turkey. "I do not like it", Trump told reporters on the White House lawn as he was returning from a trip by helicopter to Orlando, Florida.