Morsi was removed from power by the Egyptian military July 3. Human Rights Watch said five members of his administration were detained in undisclosed locations with little contact with the outside world.
"Forcibly disappearing people for months on end doesn't inspire confidence that this government intends to follow the rule of law," Sarah Leah Whitson, director of North African programs, said in a statement from New York
Morsi faces several criminal charges, including murder. Human Rights Watch said members of his government were arrested in connection with deadly clashes at the presidential palace in Cairo last December.
The organization said the government in Egypt hasn't acknowledged Morsi's aides were under arrest, a situation it says constitutes a violation of international laws.
"The prolonged enforced disappearance of anyone is a crime, pure and simple," Whitson said Sunday. "The Egyptian authorities should immediately free them unconditionally."