Pressure from the US on Saudi Arabia over the disappearance and suspected pre-meditated murder of journalist Jamal Khashoggi by Saudi agents in the country’s Istanbul consulate is starting to mount.

US senators on October 8 warned Riyadh that there could be economic consequences if Khashoggi—a columnist who wrote for titles including The Washington Post and who was self-exiled, living in Washington, DC, after leaving Saudi Arabia last year—was killed in the kingdom’s consulate in Istanbul because of his criticism of the Saudi royal family.

Republican Senator Lindsey Graham said it was “imperative” that the Saudi government give clear answers about the fate of Khashoggi. Turkish officials have briefed media outlets that he was murdered in the consulate after entering the building on October 2 to deal with documents needed to enable him to marry. Although they reportedly claim to have some information from informants in the building, they have not provided evidence to journalists. Consular officials claim Khashoggi exited the building but have not produced any proof, videotaped or otherwise, of his departure.

“If there was any truth to the allegations of wrongdoing by the Saudi government it would be devastating to the US-Saudi relationship and there will be a heavy price to be paid—economically and otherwise,” Graham of South Carolina said on Twitter. “Our country’s values should be and must be a cornerstone of our foreign policy with foes and allies alike.”

US President Donald Trump, who remained silent about the affair for several days and badly needs Saudi Arabia as a key ally in his bid to throttle the Iranian economy with heavy sanctions to force Tehran to reframe its activities in the Middle East, has now said that he is "concerned" about Khashoggi, while US Secretary of State Mike Pompeo called on Saudi Arabia to "support a thorough investigation" of his disappearance and "to be transparent about the results".

UK foreign secretary Jeremy Hunt meanwhile tweeted that he had spoken to the Saudi ambassador and that the UK "will treat the incident seriously" if reports are accurate.

Republican Senator Marco Rubio of Florida said the US should “respond strongly” if reports of the manner of Khashoggi’s death are confirmed.

Senator Bob Corker, a Republican from Tennessee who’s chairman of the Foreign Relations Committee, said he had raised the issue with the Saudi ambassador. “While we await more information, know we will respond accordingly to any state that targets journalists abroad,” Corker said on Twitter.

Senator Bob Menendez of New Jersey, the top Democrat on Foreign Relations, said on Twitter: “If true, the international community must stand together and enforce consequences.” Pompeo “must speak out forcefully against the silencing of Arab activists, dissidents and journalists,” he wrote.

Turkish President Recep Tayyip Erdogan said on October 8 he was closely following the investigation into the disappearance of Khashoggi and added that the Saudis must prove he left the consulate, and not simply state that he did.

Saudi Crown Prince Mohammed Bin Salman told Bloomberg in an interview last week that his information was that Khashoggi left the consulate shortly after entering it and that he was ready to let Turkey to search the building. On October 9 there were reports that Turkey was preparing to do that while it was searching for certain vehicles which Turkish investigators believe may have carried away Khashoggi's remains. London-based Middle East Eye reported sources as saying police were also looking into the possibility that Khashoggi's body was destroyed on-site, and maybe even burnt to ashes that could be easily disposed of.

In December 2016, Middle East Eye reported that Khashoggi had been banned by the Saudi regime from writing in newspapers, appearing on TV and attending conferences after he made remarks during a presentation at a Washington think-tank in which he was critical of Trump’s ascension to the US presidency.

Prince Mohammed’s imprisoning of opponents at home has run counter to his moves to present himself as a moderniser of Saudi Arabia.

The Khashoggi affair has the potential to greatly widen the simmering rift between regional powers Saudi Arabia and Turkey. Ankara has taken the side of Qatar over the blockading of the small nation by Saudi Arabia and other neighbours, while Turkey's rapprochement with Iran has riled the Riyadh government. Another point of contention is Turkey’s support for the Muslim Brotherhood, the Sunni Islamist organisation considered a terrorist entity by the Saudis. Turkey and Qatar are seen as the group’s primary state backers.