Following Riyadh's assertion it would retaliate, Britain, France and Germany released a joint statement saying they were treating Khashoggi's disappearance "with the utmost seriousness".
In a joint statement, the nations called for a "complete and detailed response" from Saudi Arabia.
While the kingdom has denied any involvement in Khashoggi, U.S. administration officials are said to increasingly regard the kingdom's position as untenable.
White House economic adviser Larry Kudlow declined to speculate on what Trump might do after the president promised "severe punishment" in a "60 Minutes" interview, if the USA determines that Khashoggi was indeed killed inside the Saudi consulate in Istanbul.
The controversy has also embarrassed Saudi Arabia's other traditional Western allies - many of them arms suppliers to the kingdom - and undermined efforts by the prince, Mohammed, to present himself as the modernising future of the kingdom.
Turkish sources allege he was killed by a 15-strong team of Saudi agents, but Riyadh insists that he left the consulate unharmed. The Saudi stock exchange, only months earlier viewed as a darling of frontier investors, plunged as much as 7 percent at one point Sunday before closing down over 4 percent.
The Saudi embassy in Ankara did not respond to requests for comment.
Sens. Marco Rubio and Jeff Flake, members of the Foreign Relations Committee, said Congress is prepared to move quickly and firmly if Trump fails to adequately respond to the October 2 disappearance of Khashoggi, a Washington Post contributor.
Mr Khashoggi was once an adviser to the Saudi royal family, but fell out of favour with the Saudi government and went into self-imposed exile.
"If US sanctions are imposed on Saudi Arabia, we will be facing an economic disaster that would rock the entire world", Turki Aldakhil, the general manager of Saudi Arabia-based Al Arabiya television wrote in an op-ed.
Brent oil was trading 80 cents higher on Monday at $81.25 per barrel, while United States benchmark WTI jumped 53 cents to $71.87.
Citing the disappearance of Khashoggi, on October 2.
King Salman also ordered Saudi prosecutors to begin an investigation into what happened to Mr Khashoggi, according to Reuters.
Saudi Arabia's Public Investment Fund (PIF), which hosts the conference, has tentatively committed $20 billion to an infrastructure investment planned with Blackstone Group.
The Uber investment was the PIF's first major deal in the technology start-up market, signaling its shift in strategy toward more aggressive and eye-catching foreign investments. But Khashoggi's disappearance, and suspicions he may have been targeted over his criticism of the crown prince, have led several business leaders and media outlets to back out of an upcoming high-profile investment conference in Riyadh.
Khashoggi has written extensively for the Post about Saudi Arabia, criticizing its war in Yemen, its recent diplomatic spat with Canada and its arrest of women's rights activists after the lifting of a ban on women driving.