The accident occurred before dawn Sunday on an isolated road near the highland district of Santa Lucia, about 800 kilometers (500 miles) southeast of the capital of Lima.
Police said a U.S. citizen was among those killed. She was identified by the Minneapolis Star Tribune as Meghan Sennott, 20, the daughter of one of that newspaper's photographers. He was notified about her death by U.S. Consulate officials in Lima.
She was a junior at Boston University, the paper said.
Peruvian police had earlier identified her as Negab Nechans Sennot, 31.
Also killed were an infant and a 15-year-old, both Peruvians. Police said they were both boys, but David Sucacahua, a correspondent with a local television station, Fama TV, which covered the crash, said they were girls and apparently from the same family.
Police did not immediately respond to requests for clarification of the contradictory information.
No other foreigners were listed among the dead or the injured, who were taken to hospitals in the nearby city of Juliaca.
Officials were investigating the cause of the crash, but local media attributed the crash to either the driver falling asleep or a mechanical failure that had twice prompted the driver to pull over the vehicle, which carried about 60 passengers.
Buses in Peru often are poorly maintained, and drivers frequently speed and pass each other on blind mountain curves.
Overall, 557 people were killed in rural bus accidents between July 2004 and June 2005, according to Peru's nonprofit Center for Investigation of Overland Transport.