The bus skidded off a highway in Peru's Andean Ayacucho region at dawn, overturning and plummeting down a slope of about 200 meters, Jhonny Rolando Valderrama, head of the highway protection division, told state news agency Andina.
Valderrama said the accident occurred on the Los Libertadores highway in Ayacucho in the south-central Andes and that rescue work was ongoing.
By early afternoon, a top police officer said early information indicated there were 42 passengers in the bus.
Peru's ground transportation superintendent SUTRAN said in a statement the bus belongs to local company Turismo Molina Union SAC and that it has initiated an investigation.
Company representatives did not respond to Reuters' requests for comment.
Deadly bus accidents are common in Peru, where many buses travel on precarious mountain roads or are driven by inadequately trained drivers.
At least 25 people died in April when a bus fell into a river in Peru's northern Cajamarca region, while last September at least 24 people died in a similar accident in the Anco region.