US President Donald Trump has promised a "very full report" on the murder of Saudi journalist Jamal Khashoggi on Monday or Tuesday.

"It’s a horrible thing that took place, the killing of a journalist," Trump said in Malibu on Saturday. "A very, very bad situation. Khashoggi. And somebody who was respected. It should never have happened."

"And we’ll be having a very full report over the next two days, probably Monday or Tuesday," Trump said.

He then said more definitely that there would be "a report on Tuesday." The report will address what "we think the overall impact was and who caused it, and who did it," he added.

Asked about reports that the CIA had assessed involvement by Saudi Crown Prince Mohammed bin Salman, the president said "they haven’t assessed anything yet. It’s too early."

"That was a very a premature report," he said of a Washington Post article pointing at the Saudi crown prince.

"But that’s possible - we’re going to see. But we’re going to have a report on Tuesday. And it’ll be very complete. In the meantime we’re doing things to some people that we know for a fact were involved and we’re being very tough on a lot of people," he said.

Earlier Saturday, refuting US media reports, the State Department said no "final conclusion" had been reached by the United States on who was behind Khashoggi's murder.

The Washington Post wrote that the CIA had assessed that Saudi Crown Prince Mohammed bin Salman ordered the killing that took place last month at the Saudi embassy in Istanbul.

Other major US media outlets corroborated the report, pushing the scandal back into the spotlight in Washington.

The White House said Trump spoke to CIA chief Gina Haspel and Secretary of State Mike Pompeo about the case on Saturday, but disclosed no details.

Asked about his conversation with Haspel, the president said, “I spoke with our CIA director, who’s terrific and very knowledgeable and has been studying this very closely."

Earlier the State Department denied the reports.

"Recent reports indicating that the US government has made a final conclusion are inaccurate. There remain numerous unanswered questions with respect to the murder of Mr Khashoggi," spokeswoman Heather Nauert said.

Before departing for California's wildfire zone earlier in the day, Trump told reporters: "As of this moment, we were told that he [the crown prince] did not play a role; we're gonna have to find out what they have to say."

Asked if he would eventually confirm whether the crown prince was responsible for Khashoggi's death, Trump replied, "We're taking a look at it."

"You know," he continued, "we also have a great ally in Saudi Arabia. They give us a lot of jobs and a lot of business and economic development. They have been a truly spectacular ally in terms of jobs and economic development."

Haspel briefed the president on the Khashoggi case in October following a trip to Turkey. According to the Washington Post, Haspel listened to audio recordings of Khashoggi's murder during the trip.

The Saudi government has denied the claim that the crown prince was involved in the murder of the Washington Post columnist and sharp critic of the crown prince.

But the kingdom's official version of events has changed a number of times and his body has still not been located.

After weeks of denials and under growing international pressure, Riyadh backtracked on its initial claims that Khashoggi had left the consulate alive and said last month he was killed in a "fistfight."

Later in October, the kingdom's chief prosecutor said the murder was premeditated.

This week Saudi prosecutors said that they are seeking the death penalty for five people charged over the killings.