Donald Trump has said he presumes Jamal Khashoggi is dead and that the consequences for Saudi Arabia could be “very severe” if its leaders are found to have ordered the dissident journalist’s killing.

The US president made the remarks after being briefed on the investigation by his secretary of state, Mike Pompeo, on Pompeo’s return from a trip to Riyadh and Ankara.

UK and US pull out of Saudi event over alleged murder of Jamal Khashoggi Read more

In another sign the Trump administration was dropping its defence of Riyadh and beginning to distance itself from the Saudi monarchy over Khashoggi’s suspected murder, the treasury secretary, Steven Mnuchin, announced he would not attend next week’s government-sponsored investment conference in the Saudi capital, joining a growing exodus of western corporate leaders and politicians.

Trump also ended his administration’s refusal to discuss Khashoggi’s fate. Asked whether he thought the Saudi exile was dead, the president told reporters: “It certainly looks that way to me. It’s very sad. It certainly looks that way.”

As for Washington’s response if Saudi Arabia’s rulers were found responsible for what appears to have been a grisly murder in the country’s Istanbul consulate, Trump said: “Well, it’ll have to be very severe. I mean, it’s bad, bad stuff. But we’ll see what happens.”

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The French president, Emmanuel Macron, said France had suspended political visits to Saudi Arabia in coordination with Germany, the UK and the Netherlands, pending clarification on what happened to Khashoggi. France’s finance minister, Bruno Le Maire, pulled out of the investment forum on Thursday as did the UK’s international trade secretary, Liam Fox.

On Friday, the foreign secretary, Jeremy Hunt, said Khashoggi’s alleged treatment was “completely unacceptable” and that there would be “very very serious consequences” if reports of Riyadh’s involvement were true.

Nevertheless, Hunt also showed the likely limits of Britain’s reaction by stressing that the UK-Saudi relationship was strategic and refusing to be drawn on suggestions that UK arms sales needed to be curtailed.

In Istanbul, floodlights and a drone were used in the search of the Saudi consul general’s residence after Turkish investigators were finally allowed access to the property, in the search for Khashoggi’s remains. Local news agencies in Istanbul reported that the search had been extended to two woodland areas outside the city.

Facebook Twitter Pinterest A Turkish police dog searches the back garden of the Saudi Arabian consulate general residence on 17 October. Photograph: Chris McGrath/Getty Images

The vast expanse of Belgrad Forest and farmland in Yalova province were new leads for police after investigators used surveillance footage from hundreds of cameras around Istanbul to determine vehicles owned by the Saudi consulate had travelled there late on 2 October, the day Khashoggi disappeared.

A Turkish forensics team left the consul general’s residence early on Thursday after conducting a nine-hour sweep of the building and of consular vehicles. The consulate was also searched for a second time. There appeared to be a particular focus on the garage below the consul general’s home, and parts of the property’s garden were dug up.

The consul general, Mohammad al-Otaibi, left the country with his family for Riyadh on Tuesday, after it was announced that his residence would become part of the criminal investigation.

It was not immediately clear what the search revealed, but investigators took several boxes and bags away with them. The Turkish interior ministry promised the results would be “shared with the world”.

Video recordings leaked to Turkish pro-government newspaper Sabah drew another link between a suspect in the case and the Saudi crown prince, Mohammed bin Salman, who denies any knowledge of Khashoggi’s alleged murder.

Images published on Thursday, and said to have been taken from surveillance video outside the consulate, identified a man thought to be a member of the prince’s security staff entering the building on the morning of 2 October.

The suspect is seen arriving with several other men at 9.55am. Khashoggi arrived for an appointment at 1.14pm.

Search for Khashoggi's remains focuses on consul general's house Read more

Sabah also published stills from videos that day showing the same man outside the consul general’s home and later checking out of a nearby hotel. The pictures match the profile of one of the 15 Saudi nationals photographed two weeks ago at Istanbul’s Atatürk airport. Turkish officials believe the team was behind the alleged murder inside the diplomatic mission.

A previous search of the consulate on Monday night revealed “toxic substances” and freshly painted surfaces, the Turkish president, Recep Tayyip Erdoğan, told reporters.

Khashoggi, a Washington Post columnist who moved to the US in self-imposed exile last year, has not been seen since he visited the consulate to pick up paperwork for his planned marriage.

Over the past two weeks Turkish officials have leaked increasingly shocking evidence that they say proves that the journalist, who was critical of the Saudi crown prince, was tortured and killed inside the building and his dismembered body driven to the nearby consul general’s house where it was disposed of.

Turkish and US media published details from a three-minute audio recording on Wednesday that Turkish officials described as proof that Khashoggi had his fingers severed during an interrogation. His killers then allegedly beheaded him and cut up his body with a bone saw brought by a forensics and autopsy specialist who travelled with the assassination team.

Riyadh has denied the allegations. Its official line is that Khashoggi left the Istanbul consul after concluding his business there, but has presented no evidence to support that claim.