Chad's deposed president Hissene Habre has suffered a serious heart problem while in detention in Senegal, his lawyer said on Sunday, days before he is due to go on trial for crimes against humanity in a landmark case for human rights in Africa.

Habre, 72, is undergoing tests and it is not clear whether the problems will affect the start of the trial on July 20 at a specially-appointed court in Dakar, the lawyer, Francois Serres, told Reuters.

"Habre had a serious heart problem during the night of Monday-Tuesday and the same thing again during the night of Wednesday-Thursday," Serres said, adding that his client had suffered years of high blood pressure.

Habre was detained in Senegal in 2013 after living in exile there since being overthrown in a coup in 1990.

Human rights groups hold him responsible for the torture or killing of up to 40,000 people during the eight years he led Chad, an impoverished, oil-producing nation in central Africa.

Chad's current president, Idriss Deby, served as a military chief in Habre's administration before overthrowing him in the 1990 coup.