Saudi Arabia’s announcement that suspects are in custody over the death of journalist Jamal Khashoggi is a “good first step”, Donald Trump has said, adding that he would work with Congress on a US response.

The president was speaking at a defence roundtable event in Arizona hours after Saudi Arabia claimed that Mr Khashoggi, a Washington Post contributor who was last seen on October 2, was killed in a “fistfight” at the Saudi consulate in Istanbul, Turkey.

The kingdom also said 18 suspects were in custody and that intelligence officials had been fired.

Asked by a reporter whether he thought Saudi Arabia’s explanation for Mr Khashoggi’s death was credible, Mr Trump said “I do. I do.”

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Barriers block the road leading to Saudi Arabia’s consulate in Istanbul (Lefteris Pitarakis/AP)

But he said that, before he decided what to do next, he wanted to talk to Saudi Crown Prince Mohammed bin Salman.

“Saudi Arabia has been a great ally, but what happened is unacceptable,” the president said.

Of the Saudi arrests, he added: “It’s a big first step. It’s only a first step, but it’s a big first step.”

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A security guard stands at the entrance to Saudi Arabia’s consulate in Istanbul (Lefteris Pitarakis/AP)

On Capitol Hill, politicians including Trump ally Senator Lindsey Graham expressed scepticism about the Saudi account, which was vastly different from that given by Turkish officials, who said an “assassination squad” sent by the kingdom had killed and dismembered Mr Khashoggi.

“First we were told Mr Khashoggi supposedly left the consulate and there was blanket denial of any Saudi involvement,” Mr Graham tweeted on Friday. “Now, a fight breaks out and he’s killed in the consulate, all without knowledge of Crown Prince.”

Mr Khashoggi, a prominent journalist and royal court insider for decades in Saudi Arabia, had written columns critical of Crown Prince Mohammed bin Salman and the kingdom’s direction while living in self-imposed exile in the US. He went to the Saudi consulate to obtain paperwork for his forthcoming marriage.

“The Saudi ‘explanation’ for murdering journalist and Virginia resident Jamal Khashoggi in a consulate – a fistfight gone wrong – is insulting,” tweeted Senator Tim Kaine, the 2016 Democratic vice presidential nominee. “Since the Trump Administration won’t stand up against atrocity, Congress must.”

Democratic Representative Adam Schiff of California said Saudi Arabia’s claim that Mr Khashoggi died in a brawl was not credible.

“If Khashoggi was fighting inside the Saudi consulate in Istanbul, he was fighting for his life with people sent to capture or kill him,” said Mr Schiff, the ranking member of the House Intelligence Committee.

“The kingdom and all involved in this brutal murder must be held accountable, and if the Trump Administration will not take the lead, Congress must,” he said.

In a statement on Friday night, White House press secretary Sarah Huckabee Sanders said the US will closely follow international investigations into Mr Khashoggi’s death and will advocate for justice that is “timely, transparent and in accordance with all due process”.

Earlier on Friday, Ms Sanders said Secretary of State Mike Pompeo had spoken to the Crown Prince and briefed the president and John Bolton, Mr Trump’s national security adviser.

Mr Trump dispatched Mr Pompeo to Saudi Arabia and Turkey earlier in the week to speak to officials about the case.

Meanwhile, the deputy head of Turkey’s ruling party said his country would “never allow a cover-up” over the killing of Mr Khashoggi.

Numan Kurtulmus, of the ruling Justice and Development Party, also said on Saturday that Turkey would share its evidence with the world and that a “conclusive result” of the investigation is close.

He added that he thinks “it’s not possible for the Saudi administration to wiggle itself out of this crime if it’s confirmed”.