Europe's largest economies - Britain, France and Germany - said on Sunday they were treating the case with "the utmost seriousness".
A Saudi delegation arrived in Turkey on Friday to discuss the case with Turkish authorities, following a Turkish presidential spokesperson's announcement that a joint Turkish-Saudi investigation team would be formed upon the kingdom's request.
The warning from the world's top oil exporter came after a turbulent day on the Saudi stock exchange, which plunged as much as 7 percent at one point Sunday.
And in a major new blow for the event, JPMorgan Chase CEO Jamie Dimon and Ford chairman Bill Ford also cancelled plans to attend as well as Larry Fink, the head of investment giant BlackRock, and Steve Schwarzman of Blackstone. The market clawed back some of the losses, closing down 3.5 per cent for the day.
The case has provoked an global outcry, with US President Donald Trump threatening "severe punishment" if it turns out Khashoggi was killed in the consulate and European allies calling for "a credible investigation" and accountability for those responsible.
"The kingdom affirms its total rejection of any threats and attempts to undermine it, whether by threatening to impose economic sanctions, using political pressures or repeating false accusations", the statement said. Benchmark Brent crude now trades above $80 a barrel and US gasoline prices have risen ahead of the midterm elections. But then, if they cared about such realities, the US would have cut off support to the Saudi war in Yemen years ago. Turkish officials say they believe Khashoggi was killed inside his country's consulate in Istanbul after he visited to obtain a document required to marry his Turkish fiancée.
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She was among an estimated 280 Mexico Beach residents who decided not to evacuate, administrator Tanya Castor said. At least 13 people reportedly died in Central America over the weekend as a result of storm rains and floods.
The two companies said in separate statements Sunday that they wouldn't attend the Saudi investor conference.
"The kingdom's economy has an influential and vital role in the global economy, and that kingdom's economy is only affected by the global economy".
Saudi Arabia has begun an internal investigation into the disappearance of a prominent journalist at its Istanbul consulate and could hold people accountable if the evidence warrants it, according to a Saudi official. The Saudi government denies this claim.
In Cairo, spokesman of Egypt's Ministry of Foreign Affairs Ahmed Hafez warned against exploiting this issue politically against Saudi Arabia on arbitrary charges.
USA intelligence officials have reported they are certain the interference took place in an effort to hurt Hillary Clinton's campaign.
The two countries are sharply at odds over what happened to Khashoggi, who has been critical of some policies of Saudi Crown Prince Mohammed bin Salman and Riyadh's intervention in the war in Yemen. The kingdom has poured $US100 billion into the firm's tech investing.
"There has been a kind of uncertainty surrounding the situation of the disappearance of Khashoggi which has caused the market to fall", Zidan told AFP news agency.