The Future Investment Initiative, the Saudi conference which has grabbed headlines in the past two weeks as participants have pulled out, appears to have had its official website hacked on Monday.

The website's main page was taken over with an image of Saudi Crown Prince Mohammed bin Salman wielding a sword aimed at dead journalist Jamal Khashoggi's head in a scene reminiscent of Islamic State beheadings.

Siemens's chief executive said on Monday he would not attend the three-day conference in Saudi Arabia after the country admitted that journalist Khashoggi had been killed in its consulate in Istanbul.

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The German engineering giant was one of the last companies to decide against sending its top executive to the conference after Riyadh sought to cover up Khashoggi's Oct. 2 death before admitting to a "grave mistake".

"Siemens is a reliable and committed partner of the kingdom and its VISION 2030. But for now, truth needs to be found out and justice applied," Siemens CEO Joe Kaeser said in a statement posted on his LinkedIn account outlining his motivation not to travel to the conference in the Saudi capital Riyadh.

"Time will tell how things will develop. And I do hope there will be clarity, transparency, and justice sooner rather than later," he said.

Khashoggi's killing in Istanbul had been "monstrously planned", a spokesman for Turkey's ruling AK party said on Monday. An adviser to Turkey's president rejected Riyadh's assertion that Khashoggi died in a fight, suggesting this "mocked" international opinion.

Three European powers - Germany, Britain and France - pressed Saudi Arabia to present all the facts in the killing, and Chancellor Angela Merkel said over the weekend that Germany would not export arms to Saudi Arabia while uncertainty over Khashoggi's fate persisted.

Last week ABB Chief Executive Ulrich Spiesshofer said he would also bow out of attending, following in the footsteps of others including Airbus defence chief Dirk Hoke and Deutsche Bank CEO Christian Sewing..

Senior finance and trade officials from various Western governments including the United States will also shun the forthcoming conference because of Khashoggi's killing.

Kaeser said his decision to skip the event was no condemnation of the Saudi population or a value judgment on Siemens's 2,000 local employees in Saudi Arabia and partners likeSaudi Aramco, SABIC, SEC and the Abunayyan group.

Kaeser said he wanted to consider the interests of all stakeholders, taking into account a business opportunity worth up to $30 billion by 2030 as well as the company's reputation, before taking a decision.

"But sometimes, situations develop in such a way that no one can win, where every option is wrong. The so-called Khashoggi crisis, the death of Jamal Khashoggi, is such a situation."