Human rights watchdog Amnesty International has reiterated its call for an international probe into the killing of Saudi journalist and Washington Post columnist Jamal Khashoggi.

The rights monitor made the call outside the Saudi consulate in Istanbul on Tuesday as it marked the 100th day since Khashoggi was killed there.

"It is important for the Turkish authorities to keep up their calls for an independent UN investigation, but really the whole international community needs to commit to this so that justice is done," Amnesty's Andrew Gardner told Al Jazeera.

Activists held up a symbolic street sign bearing the name of Khashoggi, a journalist and critic of the Saudi government's reform programme under the stewardship of Crown Prince Mohammed bin Salman, also known by his initials MBS.

"The murder of Khashoggi, which happened before the eyes of the whole world, should be the subject of an international investigation," Goksu Ozahishali, the organisation's campaign manager in Turkey, told reporters.

"With this symbolic activity, we are reiterating our demand of justice for Khashoggi," she added.

Several questions remain unanswered including the whereabouts of Khashoggi's remains more than three months after the murder on October 2.

Khashoggi had entered the consulate to obtain divorce papers, in order to marry his Turkish fiancee, Hatice Cengiz.

Saudi authorities initially denied involvement in Khashoggi's murder, insisting he had left the building after completing paperwork, but later admitted that a team of Saudi agents had killed him.

"After continually lying about what happened to the journalist, Saudi authorities eventually talked about Khashoggi's fate," Al Jazeera correspondent Jamal Elshayyal said.

"However, so far nobody has been punished for the crime. Turkish investigators identified 19 suspects who entered the country as part of a hit squad," he added.

Turkish and Western intelligence officials have either hinted at or directly blamed MBS for the murder but Saudi monarch King Salman left MBS's portfolios unchanged in the latest reshuffle.

The trial of those accused of murdering Khashoggi opened last week in Saudi Arabia. The prosecutor has demanded the death penalty against five of the accused whose identities have not been revealed.

"We don't believe that the Saudi authorities, who initiated a show investigation into the murder, will conduct a fair and reliable process," Amnesty's Ozahishali said.

The United Nations human rights office last week said it was unable to assess the fairness of the trial taking place, regretting that the information it had was "not sufficient" and adding that it was "against the imposition of the death penalty" on the five suspects.