Trump, on a trip to California, said the killing "should never have happened."

The report on Tuesday will explain who the US government believes killed Khashoggi and what the overall impact of his murder is, Trump said. It was unclear who is producing the report.

Trump also said the CIA finding that bin Salman was responsible for the killing was "possible."

Trump made the remarks hours after the State Department said the government was still working on determining responsibility for the death of the US-based Washington Post columnist.

"Recent reports indicating that the US government has made a final conclusion are inaccurate," State Department spokeswoman Heather Nauert said.

"There remain numerous unanswered questions with respect to the murder of Mr Khashoggi."

Nauert said the State Department will continue to seek facts and work with other countries to hold those involved in the journalist's killing accountable "while maintaining the important strategic relationship between the United States and Saudi Arabia."

Trump discussed the CIA assessment by phone with the agency's director, Gina Haspel, and Secretary of State Mike Pompeo while flying to California on Saturday, White House spokeswoman Sarah Huckabee Sanders told reporters.

The CIA had briefed other parts of the US government, including Congress, on its assessment, a development that complicates Trump's efforts to preserve ties with the key US ally.

The assessment was based largely on circumstantial evidence relating to the prince's central role in running the Saudi government.

The CIA's finding is the most definitive US assessment to date tying Saudi Arabia's de facto ruler directly to the killing and contradicts Saudi government assertions that Prince Mohammed was not involved.

Khashoggi, a critic of the crown prince, was killed in October at the Saudi consulate in Istanbul when he went there to pick up documents he needed for his planned marriage.

Both Republican and Democratic senators on Saturday urged Trump to be tough on the crown prince, with whom he has cultivated a deep personal relationship.

"Everything points to the Crown Prince of Saudi Arabia, MbS, ordering @washingtonpost journalist Jamal #Khashoggi's killing. The Trump administration should make a credible determination of responsibility before MbS executes the men who apparently carried out his orders," tweeted Senator Bob Corker, chairman of the Senate Foreign Relations Committee.

On Thursday, Saudi Arabia's public prosecutor said he was seeking the death penalty for five suspects charged in the killing of Khashoggi.

The prosecutor told reporters the crown prince knew nothing of the operation, in which Khashoggi's body was dismembered and removed from the consulate.

Trump and top administration officials have said Saudi Arabia should be held to account for any involvement in Khashoggi's death and have imposed sanctions on 17 Saudis for their role in the killing.

But they have also stressed the importance of Washington's ties with Riyadh, one of the biggest clients of the US defence industry.

Trump wants to preserve the Saudi arms deals, despite growing opposition in Congress.

"They have been a truly spectacular ally in terms of jobs and economic development," Trump said.

"As president, I have to take a lot of things into consideration."

Australian Associated Press