US Treasury Secretary Steven Mnuchin and UK International Trade Secretary Liam Fox have pulled out of an investment conference in Saudi Arabia.
It comes amid allegations the country was behind the killing of Saudi journalist Jamal Khashoggi.
Mr Khashoggi has not been seen since entering the Saudi consulate in Istanbul on 2 October, where Turkish officials allege he was murdered.
Saudi Arabia, which denies the killing, allowed investigators inside overnight.
The Dutch and French finance ministers, as well as several other politicians and business leaders, have also said they are pulling out of the summit.
However, a number of major businesses – including Goldman Sachs, Pepsi and EDF – are still intending to go despite growing pressure for a boycott.
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Mr Mnuchin announced he would pull out of the summit, following a discussion with President Donald Trump and Secretary of State Mike Pompeo.
Mr Pompeo went to Riyadh earlier this week to discuss the disappearance with Saudi King Salman.
He said he had been assured that Saudi Arabia would investigate the matter, and that he had told Mr Trump to give the country “a few more days” before deciding whether to take action.
Meanwhile, several high-profile human rights groups have demanded that Turkey ask the UN to investigate the possible killing of Mr Khashoggi.
Amnesty International, Human Rights Watch, Reporters Without Borders and the Committee to Protect Journalists, said that a “one-sided” investigation by Saudi Arabia was not good enough.
What is the summit about?
The Future Investment Initiative summit – dubbed Davos in the Desert – will take place from 23 to 25 October in the Saudi capital Riyadh.
It is being hosted by the kingdom’s Crown Prince Mohamed bin Salman to promote his reform agenda.
His plan for economic and social modernisation, called Vision 2030, aims to reduce Saudi Arabia’s dependence on oil and provide new jobs.
As a result, Mr Mnuchin’s decision to boycott the summit is seen as a substantial blow to the prince’s agenda, correspondents say.
What happened to Jamal Khashoggi?
It is not clear. Mr Khashoggi was last seen entering the Saudi consulate in Istanbul on 2 October, to pick up paperwork that would allow him to marry his fiancée Hatice Cengiz.
Turkish officials believe Mr Khashoggi was killed by a team of Saudi agents inside the consulate, with his body then removed.
Saudi Arabia has denied the claims, and initially insisted Mr Khashoggi had freely left the embassy.
It says reports on Mr Khashoggi’s death are “completely false and baseless” and that it is “open to co-operation” to find out what happened.
Is there any evidence?
The Turkish authorities say they have audio and video evidence of the killing – although these have not been made public.
Turkish media with close links to the government have published gruesome details on the alleged audio, saying screams, and the voice of the consul, Mohammed al-Otaibi, could be heard in the recording.
Yeni Safak, which is close to the government, quotes him as telling alleged Saudi agents sent to Istanbul: “Do this outside. You’re going to get me in trouble.”
Meanwhile, Turkish media say they have identified a 15-member team of suspected Saudi agents who flew into and out of Istanbul on the day of the disappearance.
However, Saudi Arabia says reports on Mr Khashoggi’s death are “completely false and baseless” and that it is “open to co-operation” to find out what happened.
On Wednesday and Thursday, Turkish investigators spent almost nine hours searching the Saudi consul’s residence, before moving on to the consulate itself about 200m (650ft) away, according to Reuters news agency.
Several vehicles with Saudi diplomatic number plates were filmed by CCTV cameras moving from the consulate to the residence just under two hours after Mr Khashoggi entered the consulate.
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How have other countries reacted?
Saudi Arabia is a key ally to many Western countries, and, as one of the world’s biggest oil exporters, has significant influence on the world stage.
However, several politicians have pulled out from next week’s summit including International Monetary Fund chief Christine Lagarde.
The Foreign Minister of the Netherlands, Stef Blok, said he was pulling out because “the disappearance of Khashoggi is a very serious matter. Saudi Arabia has not yet been able to provide any clarification.”
A spokesman for Dr Fox said: “The UK remains very concerned about Jamal Khashoggi’s disappearance… those bearing responsibility for his disappearance must be held to account.”
Mr Trump has previously vowed to inflict “severe punishment” on Saudi Arabia if it was behind Mr Khashoggi’s killing – but also said he is against cancelling military contracts with Riyadh.
He told AP news agency he was unhappy with the global condemnation of Saudi Arabia, saying it was being treated “guilty until proven innocent”.
Russian President Vladimir Putin has said it is a pity that Mr Khashoggi has gone missing, but that Russia cannot damage relations with Saudi Arabia without hard facts.
Who is Jamal Khashoggi?
Mr Khashoggi is a prominent journalist who has covered major stories for various Saudi news organisations.
He served as an adviser to top Saudi officials, but later fell out of favour with the government.
He went into self-imposed exile in the US last year, and wrote a monthly column in the Washington Post.
On Thursday, the Washington Post published Mr Khashoggi’s latest column – a call for press freedom across the Arab world.
The newspaper said the column had been submitted by Mr Khashoggi’s translator the day after he was reported missing.
It had initially held off from publishing the column, but decided to go ahead after accepting Mr Khashoggi was not going to return safely.
In his column, Mr Khashoggi criticised the state of press freedom in the Arab world, saying it left Saudis “either uninformed or misinformed”.
Government moves to suppress journalists “no longer carry the consequence of a backlash from the international community”, he wrote. “Instead, these actions may trigger condemnation quickly followed by silence.”
Jamal Khashoggi disappearance: The key events
- 03:28: A private jet carrying suspected Saudi agents arrives at Istanbul airport. A second joins it late afternoon
- 12:13: Several diplomatic vehicles are filmed arriving at the consulate, allegedly carrying some of the Saudi agents
- 13:14: Mr Khashoggi enters the building, where he is due to pick up paperwork ahead of his marriage
- 15:08: Vehicles leave the consulate and are filmed arriving at the nearby Saudi consul’s residence
- 21:00: Both jets leave Turkey by 21:00
- Turkish government announces Mr Khashoggi is missing, thought to be in the consulate
- Saudi Arabia says he left the embassy
- Turkish officials tell the BBC they believed Mr Khashoggi was killed at the consulate. This is later strongly denied by Saudi Arabia
- Turkish officials tell BBC Arabic they have audio and video evidence of the killing . The existence of such tapes had previously been reported by local media
15 and 17-18 October
- Forensic teams carry out searches of consulate