His disappearance sparked a global outcry and prompted some USA lawmakers to call for harsh action against Riyadh. In a statement, his spokesperson Stephane Dujarric said: "The Secretary General is deeply troubled by the confirmation of the death of Jamal Khashoggi".
Eighteen people have been arrested and countries such as Turkey have accused Saudi Arabia of a state-sponsored killing.
None of the Saudi statements give any clue as to what happened to Khashoggi's remains. "If the President will not stand up to Saudi Arabia, then we in Congress must stand strong for our nation, our values and journalists throughout the world", Costa said.
What has the USA said about this?
He said on Thursday he believes Khashoggi is dead and that the USA response to Saudi Arabia will likely be "very severe" but that he wanted to get to the bottom of what happened.
In related developments, senior government officials from the United States, France, Britain and the Netherlands cancelled out of an investment conference in Saudi Arabia amid questions over the kingdom's involvement in Khashoggi's disappearance.
Turkish authorities said days ago, that they had an audio recording of the incident in which they say Khashoggi was interrogated, which led to a struggle, which led to Jamal Khashoggi's death.
Asked by a reporter whether he thought Saudi Arabia's explanation for Khashoggi's death was credible, Trump said, "I do". Saudi King Salman also had a plan to restructure the kingdom's intelligence services.
But, the disappearance of Khashoggi has tarnished Prince Mohammed's reputation and deepened questions about his leadership.
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In all, 33 MPs, including Mr Chishti, have visited oil-rich Saudi Arabia since March 2015, with the trips worth more than £200,000.
Trump has repeatedly pointed out that Khashoggi wasn't a US citizen and on Monday floated the notion that "rogue killers" may have been involved.
Saud Qahtani, a powerful adviser to Prince Mohammed, also was sacked.
"If Khashoggi was fighting inside the Saudi consulate in Istanbul, he was fighting for his life with people sent to capture or kill him".
"The termination of the two officials was done by the king, not his heir apparent, the crown prince, which could be an indication the aging monarch is cleaning up an extremely ugly mess created by the younger man."
"The level of detail is very damning of the hit squad, and [their reported identities] show how close they are to the crown prince". The monarch also relieved deputy intelligence chief Gen. Ahmed al-Assiri.
"There were no orders for them to kill him or even specifically kidnap him", said the official, speaking on condition of anonymity and adding that there was a standing order to bring critics of the kingdom back to the country.
The person said Mohammad bin Salman had no knowledge of the operation.
This is the latest claim made by the kingdom, which earlier said Khashoggi had left the consulate alive and well. He did not disclose the whereabouts of his body.