Secretary of State Mike Pompeo, hurriedly sent to Riyadh, expected to get more clarity during talks with Saudi leaders Tuesday.
Turkish officials have said they believe he was killed, a claim Saudi Arabia has denied.
Three high-profile Wall Street CEOs reportedly attempted to persuade Saudi officials to push back Saudi Arabia's Future Investment Initiative conference.
Donald Trump says he has spoken to King Salman bin Abdulaziz Al Saud for about 20 minutes about the Jamal Khashoggi case.
"I heard that report, but nobody knows if it's an official report", Trump told reporters, without elaborating. "Determining what happened to Jamal Khashoggi is something of great importance to the president", state department spokeswoman Heather Nauert said on Monday as Pompeo prepared to take off from Washington.
Earlier on Monday, Turkish and Saudi officials arrived at the Saudi Consulate in Istanbul after the start of a joint probe into the missing journalist.
It was the first time they had been allowed to enter the building since the journalist's disappearance following disagreements between the two sides over the terms of any search.
Previously, Saudi authorities had maintained Khashoggi left the consulate the same afternoon of his visit, but provided no evidence to support the claim.
"The king firmly denied any knowledge of it", Trump told reporters after speaking with King Salman.
He made that point again and again as he left the White House, telling reporters, "All I can do is report what he told me". "It sounded to me like maybe these could have been rogue killers".
King Salman emphasised the importance of the Turkey-Saudi relationship and said no-one should be able to "undermine the strength of this relationship", Saudi's official media reported.
October 14: In an interview aired on CBS's "60 Minutes," Trump addresses Khashoggi's disappearance. But he has said repeatedly that he does not want to halt a proposed $110 billion arms sale to Saudi Arabia - as some in Congress have said he should - because it would harm the USA economically.
"The objective of (Sunday's) message broadcasted by the Saudi national media is twofold: one goal is to satisfy the domestic public opinion that Saudi Arabia and its royal family will not be humiliated and the other goal is to send a message to the US", Mitreski, researcher at the University of Sydney, told AFP.
"The world is watching, the world is talking". Rubio said U.S. Treasury Secretary Steven Mnuchin should also withdraw, but White House economic adviser Larry Kudlow said the Treasury chief is still planning to go.
Three major lobbying firms in Washington - Glover Park Group, the Harber Group and BGR Group - have also cut ties to Saudi Arabia amid Khashoggi's disappearance.
A key USA lawmaker, Republican Senator Marco Rubio, told CNN on Sunday that if Saudi agents "went medieval" on Khashoggi, "that would be an outrage". "We are concerned. On the face of it it's upsetting".
Riyadh's most recent comments have focused on having no knowledge of any killing or denying any such order had been given.