Turkish investigators are likely to find out what happened to the body of Jamal Khashoggi before long, a senior official said on Saturday, after Saudi Arabia admitted for the first time that the journalist had been killed in its Istanbul consulate.

The official, who declined to be identified, also told Reuters that Khashoggi’s DNA samples were being procured from Turkey, meaning that investigators would not need to ask Saudi Arabia for samples at the moment.

Saudi Arabia said on Friday that Khashoggi had died in a “fistfight” inside its Istanbul consulate – Riyadh’s first acknowledgement of his death after two weeks of denials that it was involved in his disappearance.

The Saudi regime also announced a purge of senior officials including Saud al-Qahtani, an influential adviser to Crown Prince Mohammed bin Salman, and General Ahmed al-Asiri, a senior intelligence official. Both men have been fired. Eighteen Saudi nationals were said to have been arrested.

The announcement, which cited preliminary findings from an official investigation, was made on state television. The purge appeared to be aimed at insulating the crown prince and protecting his position. It was reported that he would remain overall head of intelligence.

Khashoggi, a critic of the crown prince, went missing after entering the consulate to obtain documents for his upcoming marriage. Days later, Turkish officials said they believed he was killed in the building, an allegation that Saudi Arabia had, until now, strenuously denied.

A follow-up statement, released by the Saudi Arabian ministry of foreign affairs, claimed that discussions between Khashoggi and Saudi officials at the consulate “did not go as required and escalated negatively which led to a fight between them … and led to his death”. It also claimed that officials, referred to as “suspects”, were involved in a “cover-up”.

Speaking to reporters on Friday, Trump said Saudi Arabia’s announcement on the circumstances of Khashoggi’s death was credible and a “good first step” but that what happened was “unacceptable”. He also said he prefers that any sanctions against Riyadh not include canceling big defense orders.

The Trump administration has stressed it cannot afford to lose Saudi Arabia as a strategic partner. But it faces a sceptical Congress which may demand more convincing explanations of what happened to Khashoggi on 2 October.

The claim that Khashoggi, 59, died in a fight with Saudi officials who greeted him at the Istanbul consulate, prompted derision elsewhere.

“To say that I am skeptical of the new Saudi narrative about Mr Khashoggi is an understatement,” the Republican senator Lindsey Graham said in a series of tweets.

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

The California congressman Adam Schiff, the ranking Democrat on the House intelligence committee, said Saudi Arabia’s claim that Khashoggi was “killed while brawling with a team of more than a dozen dispatched from Saudi Arabia is not credible”.

Schiff said if Khashoggi was fighting inside the consulate, he was “fighting for his life with people sent to capture or kill him”. If Trump’s Republican administration won’t hold Saudi Arabia accountable for Khashoggi’s death, Congress will, he added, according to Reuters.

Leaks from the Turkish authorities and independent reporting has shown that the kingdom’s most senior forensics expert was among a 15-man team sent from Riyadh on 2 October ahead of Khashoggi’s scheduled visit to the consulate.

They are reported to have brought a bone saw with them and the forensics expert, Salah Muhammed al-Tubaigy, is said to have been recorded telling others to listen to music on headphones while he dismembered the body.

It will also be hard to convince global opinion that the crown prince, widely known by his initials, MBS, did not give the order. Several of the 15-strong squad sent to Istanbul were from his personal security staff.

The credibility of the Saudi court is already stretched to breaking point. From the day of Khashoggi’s disappearance until the early hours of Saturday morning, the official line from Riyadh was that the exiled writer, a US resident who wrote for the Washington Post, left the consulate before disappearing.

The question for western powers is whether this explanation is seen as a necessary and sufficient grubby deal to bring a disastrous episode to a close, or whether punitive sanctions will be required.

The White House press secretary, Sarah Sanders, said in a statement that the US “acknowledges the announcement from the Kingdom of Saudi Arabia that its investigation into the fate of Jamal Khashoggi is progressing and that it has taken action against the suspects it has identified thus far”.

She added: “We will continue to closely follow the international investigations into this tragic incident and advocate for justice that is timely, transparent, and in accordance with all due process. We are saddened to hear confirmation of Mr Khashoggi’s death, and we offer our deepest condolences to his family, fiancée, and friends.”

Samantha Power, the former US ambassador to the United Nations, said in a tweet: “They don’t get it. Shifting from bald-face lies (‘Khashoggi left consulate’) to faux condemnation (of a ‘rogue operation’) to claiming the wolf will credibly investigate what he did to the hen … will convince nobody.”

A spokesperson for the UN secretary general, Antonio Guterres, said he was “deeply troubled” by the confirmation of Khoshoggi’s death, and stressed the “need for prompt, thorough, transparent investigation” into the circumstances, and full accountability for those responsible.