"In accordance with criminal laws... (Aung San Suu Kyi) has been kept in solitary confinement in prison" since Wednesday, Zaw Min Tun said in a statement, AFP reported.
Since her ouster in a coup last year, Suu Kyi had been under house arrest at an undisclosed location in Naypyidaw, accompanied by several domestic staff and her dog, according to sources with knowledge of the matter.
The Nobel laureate, 77, left those premises only to attend hearings for her trial in a junta court that could see her handed a prison sentence of more than 150 years.
Suu Kyi's lawyers have been banned from speaking to the media and journalists barred from her trial.
Under a previous junta regime, she spent long spells under house arrest in her family mansion in Yangon, Myanmar's largest city.
She has already been convicted of corruption, incitement against the military, breaching COVID-19 rules and breaking a telecommunications law, with a court sentencing her to 11 years so far.