(Recasts lead and top paragraphs with Trump quotes about CIA assessment)

By Julia Harte and Steve Holland

WASHINGTON/MALIBU, California, Nov 17 (Reuters) - President Donald Trump on Saturday called a CIA assessment blaming Saudi Crown Prince Mohammed bin Salman for the killing of Saudi journalist Jamal Khashoggi "very premature" and said he will receive a complete report on the case on Tuesday.

Trump, on a trip to California, said the killing "should never have happened." The report on Tuesday will explain who the U.S. 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 Khashoggi, a U.S.-based Washington Post columnist.

"Recent reports indicating that the U.S. government has made a final conclusion are inaccurate," State Department spokeswoman Heather Nauert said in a statement. "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 U.S. government, including Congress, on its assessment, sources told Reuters on Friday, a development that complicates Trump's efforts to preserve ties with the key U.S. ally.

A source familiar with the CIA's assessment said it 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 U.S. 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.

As lawmakers push legislation to punish Saudi Arabia for the killing, 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.