Agnes Callamard, the special rapporteur on extrajudicial executions mandated by the U.N.-backed Human Rights Council, also put an onus on the five permanent U.N. Security Council countries.
Callamard said in a statement Thursday that the Saudi government invited representatives from the five countries to attend some court hearings.
She said China, France, Britain, Russia and the United States "risk being participants in a potential miscarriage of justice" and could be "complicit" if the trials turn out to involve violations of human rights law.
Callamard is leading a human rights probe into the Khashoggi's killing at the Saudi consulate in Istanbul in October.
She said the whereabouts of Khashoggi's remains and information about those initially arrested over the killing should be made public.
The U.N. human rights office said 11 people are on trial, five of whom face the death penalty, and "it appears" 21 people were initially detained by Saudi authorities.
Callamard has been seeking authorization to travel to Saudi Arabia as part of her investigation.