Ousman Sonko, a former interior minister of Gambia suspected of human rights abuses, will have to remain in Swiss detention until at least the end of next month.
The Federal Criminal Court on Thursday rejected an appeal by Sonko's lawyers against a decision by the Office of the Attorney General last month to expand criminal proceedings against Sonko to investigate allegations of crimes against humanity.
Judges said their decision was based on independent reports by United Nations special observers on Gambia.
However, the latest ruling can still be challenged at the Swiss Supreme Court.
Sonko, who sought asylum in Switzerland last year was arrested in January. He allegedly commanded a special squad known for its brutality against citizens in the west African state of Gambia.
The Swiss authorities have extended Sonko’s detention twice in the past.
Trial International, an international justice organisation, accused Sonko of having personally taken part in what it described as torture between 2006 and 2016. Sonko served under ousted Gambian leader Yahya Jammeh.
swissinfo.ch with agencies/ug