Trump, in comments in the Oval Office, told reporters he had raised Khashoggi's case with Saudi Arabia "at the highest level" and a couple of times in recent days. Although his opinions had angered certain people, he said, the tensions between himself and Saudi Arabia did not amount to hate, grudges or threats.
But in his first extensive remarks about Khashoggi's case, Trump warned it is "a very sad situation".
"We can not let this happen, to reporters, to anybody", Trump said.
The Tennessee Republican, who chairs the Senate Foreign Relations Committee, says he has also spoken with the Saudi ambassador to the U.S.
"We don't know what has happened to him".
This video grab made on Wednesday from CCTV footage obtained from Turkish news agency DHA shows Saudi journalist Jamal Khashoggi (right) arriving at the Saudi Arabian consulate in Istanbul on Oct 2, 2018.More news: MIT alumnus William Nordhaus wins Nobel Prize in economic sciences
In Istanbul, Khashoggi met his fiancee, Hatice Cengiz, a PhD student who had met Khashoggi at a seminar.
He said Iran's sales had fallen by 700,000 barrels per day since the United States announced in May that it was pulling out of the 2015 nuclear deal with Iran and reimposing sanctions on its oil industry.
Pence did not indicate that either Turkey, which has launched an intensive investigation into Khashoggi's disappearance, or Saudi Arabia has sought US assistance.
He was also a U.S. resident living in self-imposed exile in Washington. Nauert said Secretary of State Mike Pompeo and other USA diplomats have been engaged on the issue and that Trump himself - he said Tuesday he "knows nothing" about what happened to Khashoggi - intends to speak to the Saudis. "But as I said yesterday, the free world deserves answers". Frankly, because it's a reporter, you could say in many respects, it brings it to a level. Saudi Arabia still insists it's all baseless. "I very much doubt we could pass another one", Corker said.
Mr Khashoggi had written a series of columns for the Washington Post that were critical of Saudi Arabia's assertive Crown Prince Mohammed bin Salman, who has led a widely publicised drive to reform the Sunni monarchy but has also presided over the arrests of activists and businessmen.
The same day, Turkish media published images of an alleged 15-member Saudi "assassination squad" and video of suspicious movements at the Saudi Consulate in Istanbul following Khashoggi's disappearance, putting new pressure on the kingdom amid growing worldwide concern for the writer.
The editorial page editor of the Washington Post told the Daily Beast that his paper was aware that Khashoggi was looking to formally move into the advocacy world.More news: Ousted Guardians of the Galaxy director switches camps, tied to new DC Comics film
She called on Trump to "help shed light" on the journalist's disappearance.
In his last interview three days before his disappearance, Khashoggi said that he did not think he would return to Saudi Arabia. Sabah newspaper published the names and years of birth of 15 Saudis it said arrived at Istanbul's Ataturk airport on October 2.
The Sabah newspaper, which is close to President Recep Tayyip Erdogan, identified the team members, including several alleged security officials, and published photos of each of them, apparently taken at passport control.
Footage also showed some of the Saudis arriving in Istanbul after the first plane landed before 0030 GMT on October 2 and the men later checking into a hotel.
The announcement came as a surveillance image surfaced of Jamal Khashoggi walking into the Saudi Consulate in Istanbul, just before he disappeared.
"Silence, denials and delays are not acceptable".More news: Google Pixel 3 derided for notch by netizens
Turkey has said Saudi authorities gave officials the greenlight to search the consulate but it has not yet taken place.