LIMA -- As many as 25 people were killed and another 13 injured after a passenger bus plunged into a 300-meter-deep ravine in north Peru on Sunday night, authorities said on Monday.

The interprovincial bus was carrying some 50 passengers when it crashed around 10:10 pm local time on the highway to Celendin, in Cajamarca department, the head of police in Celendin province, Hugo Uriarte, told local media.

He added that the bus departed with 35 passengers on board but picked up more travelers along the route.

Rescue operations continue but have been complicated by the depth and inaccessibility of the ravine, Uriarte said.

The head of Communications for Celendin province, Jaime Herrera, told state-run TV Peru that the bus ended up in the Sendamal River, making it likely that several passengers were swept away by the flow.

Some of the survivors, including the driver, sustained injuries that were very serious and have been taken to hospitals in Celendin and Cajamarca.

Authorities are gathering information from the survivors to determine the cause of the accident.

The city of Celendin conveyed its condolences to the victims' families in a statement, and called on authorities to carry out an "exhaustive investigation to determine responsibilities." It also declared a 72-hour province-wide period of mourning.