The United States (US) will make final conclusions by early next week over the killing of Jamal Khashoggi, President Donald Trump said Saturday, following reports that the Central Intelligence Agency (CIA) had held Saudi Crown Prince Mohammed bin Salman responsible.

Saudi Arabia has repeatedly changed its official narrative of the October 2 murder, first denying any knowledge of Khashoggi's whereabouts and later saying he was killed when an argument degenerated into a fistfight.

Earlier this week, a Saudi prosecutor exonerated the crown prince of involvement in the brutal murder.

Speaking to reporters in Malibu, California after surveying damage from wildfires, Trump said, "We'll be having a very full report over the next two days, probably Monday or Tuesday."

State Department Spokeswoman Heather Nauert earlier said reports indicating the US had already made final conclusions in the case were "inaccurate".

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

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

"In the meantime, we will continue to consult Congress, and work with other nations to hold accountable those involved in the killing of Jamal Khashoggi."

She noted that Washington had already taken "decisive measures" against individuals, including visa and sanctions actions.

The remarks appeared to contradict reports that the CIA had determined Saudi Arabia's powerful Crown Prince Mohammed bin Salman ordered the killing of Khashoggi, a vocal critic.

'Truly spectacular ally'

The killing and the international uproar it triggered has frayed ties between Washington and longtime ally Riyadh, which has sought to end discussion of the murder and rejected calls for an international investigation.

But ahead of a briefing by his secretary of state and CIA director, Trump demurred when asked about possible retaliation against Riyadh.

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

"And I also take that ─ you know, I'm president ─ I have to take a lot of things into consideration."

The Washington Post, which broke the story, said the CIA found that 15 Saudi agents flew on government aircraft to Istanbul and assassinated Khashoggi in the Saudi consulate.

Khashoggi, a Post columnist, had gone to the consulate to obtain documents necessary to marry his Turkish fiancee.

European Union foreign affairs chief Federica Mogherini meanwhile reiterated calls for a "thorough, credible and transparent" probe into Khashoggi's killing.

"The need remains to shed full clarity on the circumstances surrounding this horrendous crime as well as to ensure accountability for all those responsible for it," she said in a statement.

Reiterating the EU's categorical opposition to the death penalty, Mogherini said, "We will continue to stress that the kingdom of Saudi Arabia must put in place measures to ensure that something like this can never happen again."

"In due course, the EU and its member states will consider how they can act together towards appropriate measures against those responsible, in support of the rules based international system," she added.

In the latest version of the events presented by the Saudi prosecutor on Thursday, a 15-member squad was formed to bring Khashoggi back from Istanbul "by means of persuasion" ─ but instead ended up killing the journalist and dismembering his body in a "rogue" operation.