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 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 Khashoggi’s death was credible, Trump said “I do. I do”.

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,” Trump 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.”

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 Khashoggi.

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

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 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 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 Khashoggi’s death and will advocate for justice that is “timely, transparent and in accordance with all due process”.

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

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

Two days after Khashoggi vanished into the Saudi Consulate in Istanbul, the Washington Post published a column featuring his byline and the headline “A missing voice”. The space below it was blank.

In a final column for the Post, which the newspaper said it received from his assistant on October 3 and published on October 17, Khashoggi warned that governments in the Middle East “have been given free rein to continue silencing the media at an increasing rate”.

He noted that some Middle East leaders were blocking internet access so they could tightly control what their citizens can see.

“The Arab world is facing its own version of an Iron Curtain, imposed not by external actors but through domestic forces vying for power,” he wrote.

Born into a family of wealth and connections – he was the nephew of Saudi arms dealer Adnan Khashoggi and a cousin of Diana, Princess of Wales’s boyfriend, Dodi Fayed – he was a voice of moderation in a kingdom at war with terrorism in the aftermath of the September 11, 2001, attacks in the United States.

He spent years explaining its policies to outsiders, but made himself unpopular at home, saying the Saudi-led intervention in Yemen “would validate” those who compared the kingdom’s actions to what Russia and Iran were doing in Syria. He also was critical of Riyadh’s diplomatic break with Qatar.

After Khashoggi criticised the kingdom’s celebration of Donald Trump’s election as US president in 2016, a royal court official who was close to him advised him to stop tweeting and publishing stories, a sign that his opinion was no longer welcome.

Khashoggi went into a self-imposed exile, moving to Washington in 2017, writing regular columns for the Post and pursuing pro-democracy projects.

The Crown Prince’s crackdown intensified after Khashoggi left, reaching some of his friends and associates.

Sherif Mansour, of the Committee to Protect Journalists, said Khashoggi was one of the few Saudis who helped track news of missing or detained journalists and activists.

“Saudi was always a black hole in terms of information, and now, after Jamal’s case, it is even harder to get any. It is up to us now to make sure the risks he took on those journalists’ behalf were not in vain,” he said.