The toll in the accident "has risen to 11 dead. The injured are in stable condition," Fouad Kharmaz, a health ministry official in Khemisset province, told AFP.
The crash happened Wednesday, when local authorities initially reported five dead and 27 injured.
Nine of the 11 dead were female farm laborers, Kharmaz said.
Their minibus slammed into a tree after the driver lost control in the rural town of Brachoua, local officials said.
As in the rest of the Maghreb, road accidents are common in Morocco, where many poorer citizens use coaches and minibuses to travel in rural areas.
In August, 23 people were killed and 36 injured when their bus overturned on a bend east of Morocco's economic capital Casablanca, one of the deadliest such accidents in recent years.
An average of 3,500 road deaths and 12,000 injuries are recorded annually in Morocco, according to the National Road Safety Agency, with an average of 10 deaths per day.
The figure last year was around 3,200.
Since the worst bus accident in the country's history, which left 42 dead in 2012, authorities have set out to halve the mortality rate by 2026.
Morocco Crash Kills Eleven
RABAT - Eleven people, mostly agricultural workers, have died in a road accident in Morocco, the health ministry said Thursday.