Saudi Arabia officials said that he had left the building, but have not provided evidence.
In an interview with U.S. 60 Minutes, Mr Trump said the Saudis denied involvement "in every way you can imagine" when the crown prince spoke to his son-in-law Jared Kushner.
"What was explained to us is this: "He was killed, make your funeral preparations", Kislakci said.
"There are other things we can do that are very, very powerful, very strong and we'll do them", he added, without saying what those measures might be.
He said he and his wife, Melania, expect to invite Cengiz to the White House soon.
President Trump said he would meet with Mr Khashoggi's family. "That's a bad situation. We can not let this happen, to reporters, to anyone".
The US leader said on Wednesday that he had talked to the Saudi authorities "at the highest level" about Khashoggi.
The US, where Khashoggi lived and worked in recent years, is also keen on solving his disappearance.
"I'm not happy about it", Trump said of Khashoggi's disappearance.
"Frankly I think that would be a very very tough pill to swallow for our country".
Asked if he believed it was the Saudis, Trump responded: 'I would rather not say, but the answer is yes, ' he said. "It is also likely that an worldwide stance will be formed against Saudi Arabia which is a law-breaking government and bin Salman will be sued by the global Court of Justice (ICJ)", Mujtahid wrote on his twitter page on Tuesday.
'Secretary of State Mike Pompeo then had a follow up call with the Crown Prince to reiterate the United States request for information. "I think she's in shock, now even more", her friend said.
The president had been criticised for his mild response to the disappearance of the journalist, with members of Congress imploring his administration try to get to the bottom of the disappearance.
But both the White House and the kingdom are under mounting pressure as concern grows over the fate of the veteran journalist, who has not been seen since he entered the Saudi Consulate in Istanbul on October 2.
Trump says nobody knows exactly what happened and expressed hope that Khashoggi is not dead. Local media reported that Turkish employees at the consul's residence were abruptly told to leave on the day Khashoggi disappeared. Cengiz did not see him re-emerge, although Saudi officials said he left the consulate shortly after visiting.
On Wednesday, State Department spokesman Robert Palladino said the United States "had no advance knowledge" of such a plan.
After the Turkish government informed the American officials that it has audio and video recordings proving that Jamal Khashoggi, the Saudi journalist was killed inside the Istanbul consulate, US President Donald Trump Saturday promised to take "severe punishment" against Saudi Arabia if the reports were indeed true.
An official leaves the Saudi Arabia consulate in Istanbul, Oct. 7, 2018. He had been living in the United States since past year, in self-imposed exile, partly because of the rise of Saudi Crown Prince Mohammed bin Salman, who has shown little tolerance for criticism.
Trump, who took his first overseas trip as US president to the kingdom and whose son-in-law Jared Kushner has close ties to Prince Mohammed, said Tuesday he had not yet talked to the Saudis about Khashoggi, "but I will be at some point", without elaborating.
Aksam said they left their hotel later in the morning.
Earlier on Wednesday, Turkish media outlets published CCTV footage which they say shows evidence of a plot linked to Mr Khashoggi's disappearance.