Amnesty International said early Saturday that the "impartiality" of a Saudi probe into the case would remain in question after the kingdom admitted that Khashoggi had been killed inside Riyadh's consulate in Istanbul.
Earlier on Saturday, Riyadh announced the arrest of 18 Saudis and the sacking of two top aides of Crown Prince Mohammed bin Salman, as part of its "probe" into the killing.
"All along we were concerned about a whitewash, or an investigation by the entity suspected of involvement itself," the London-based rights group's Rawya Rageh said.
"The impartiality of a Saudi investigation would remain in question," she added.
Rageh said human rights organizations have underlined the need for "an impartial and independent investigation by the UN to find out what happened and ensure justice" for Khashoggi.
The remarks came after Saudi Attorney General Sheikh Saud al-Mojeb said that Khashoggi had died after "discussions" at the consulate turned into "a brawl and a fist fight."
The admission of Khashoggi's murder at the hands of Saudi officials followed more than two weeks of denials by the kingdom of any involvement in Khashoggi's disappearance.
"God have mercy on you my love Jamal, and may you rest in Paradise," Khashoggi's fiancée Hatice Cengiz tweeted after the Saudi confirmation of the journalist's death.
Reporters Without Borders (RSF) also called on the international community to keep up pressure on Saudi Arabia to get the whole truth about Khashoggi's case.
RSF Secretary General Christophe Deloire on Saturday blasted Riyadh for imprisoning, kidnapping and even killing journalists.
"Any attempt to get rid of the pressure on Saudi Arabia and to accept a compromise policy would result in giving a 'license to kill' to a Kingdom that puts in jail, lashes, kidnaps and even kills journalists who dare to investigate and launch debates," he tweeted.
"After the recognition of Khashoggi's death, we expect a determined, constant and powerful pressure to be kept on Saudi Arabia in order to get the whole truth on the case and the release of Saudi Arabian journalists (who have) been condemned to crazy and horrible sentences."
However, the United Arab Emirates, which is Saudi Arabia's key ally in a devastation war on Yemen, hailed the kingdom for its response to Khashoggi's death.
"The United Arab Emirates welcomes the decisions and the directives by King Salman," regarding the Khashoggi affair, state news agency WAM said.
Khashoggi, an outspoken critic of the Riyadh regime, vanished at the Saudi mission in Istanbul on October 2.
Audio recordings from within the consulate suggested that a team of 15 Saudi agents, among them a forensics expert, had been waiting when Khashoggi walked into the building to obtain documents for his upcoming marriage. The agents were also reported to have brought a bone saw with themselves.
Media reports said that several of the Saudi hit squad were from Crown Prince Mohammed bin Salman's personal security staff.
Amid international outcry over Khashoggi's brutal murder, the Saudi public prosecutor said 18 nationals had been detained in connection to the probe.
Saudi state media also reported that Saudi King Salman bin Abdulaziz Al Saud had ordered the dismissal of two senior officials, deputy intelligence chief Ahmed Asiri and Saud al-Qahtani, a royal court advisor seen as the right-hand man to bin Salman.
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