
Ieng Sary, 82, was arrested with his wife, former Khmer Rouge minister for social action Ieng Thirith, last year amid fears they could flee the country, destroy crucial evidence and incite public unrest.
The Cambodia Daily newspaper quoted co-investigating judge You Bunleng as saying that the conditions of Ieng Sary and Ieng Thirith's detentions would be outlined after this week's three-day Water Festival public holiday.
Ieng Sary is one of five former Khmer Rouge leaders due to be tried before an UN-backed court, which was established in 2006.
The leaders will face trial for their roles in atrocities committed during the Khmer Rouge's 1975 to 1979 reign, in which up to two million people died through execution, starvation or overwork.