Hissene Habre has been accused of widespread murder and torture
Belgium has lodged a case at the International Court of Justice seeking to compel Senegal to prosecute former Chadian President Hissene Habre.
Mr Habre, who is accused of crimes against humanity, has lived in Senegal since being removed from power in 1990.
He is accused of killing and torturing tens of thousands of opponents during his eight-year rule, charges he denies.
The African Union has told N'Djamena to prosecute him, but it has reportedly said the procedure would be too costly.
In August, Senegalese Justice Minister Madicke Niang said 18bn CFA francs ($35m) were likely to be needed to fund the trial.
Mr Habre, sometimes dubbed "Africa's Pinochet", settled in Senegal after he was deposed in 1990 by Chadian President Idriss Deby.
A commission of inquiry has said Mr Habre's government was responsible for some 40,000 politically motivated murders and 200,000 cases of torture. He denies knowledge of the crimes.
Belgium has been pushing to have Mr Habre put on trial since a Belgian national of Chadian origin and several Chadian nationals filed complaints in Belgian courts between November 2000 and December 2001.
Belgium's "universal jurisdiction" law allows prosecutions for crimes against humanity wherever they are committed.
In September 2005, Belgium issued an international arrest warrant for the former Chadian leader relating to the activities of his intelligence service, which is accused of arbitrary arrests, mass murder and systematic torture.
Mr Habre was arrested by the Senegalese authorities two months later, but a court ruled that he could not be tried there and rebuffed Belgium's extradition request.
Mr Habre's forces employed a scorched earth strategy in southern Chad
The African Union ordered Senegal to put Mr Habre on trial in 2006, but since then N'Djamena has said it is having financial difficulties.
"Senegal's failure to prosecute Mr Habre, if he is not extradited to Belgium to answer for the acts of torture that are alleged against him, violates the [UN] Convention against Torture," Belgium said in documents filed on Thursday, according to the International Court of Justice (ICJ).
The ICJ said Belgium was seeking an order compelling Senegal to put Mr Habre on trial, or else extradite him "so that he can answer for his crimes".
Belgium also wants the court to take to take "all the steps within its power to keep Mr Habre under the control and surveillance of the judicial authorities of Senegal", so that he can not flee Senegal.
Human Rights Watch (HRW) said Belgium's lawsuit move was an important step towards ensuring Mr Habre did not escape justice.