At least 30 people were killed Monday after a passenger bus plunged into an 800-meter-deep abyss in the southern Bolivian town of Yocalla, police said.

"There are at least 30" deceased who have been counted and are being transferred to the morgue, Colonel Victor Benavides told AFP.

The accident occurred on a narrow two-way road between the cities of Potosi and Oruro, with the route running along a ravine that is almost half a mile deep.

Local news outlet Unitel reported that 15 people had been injured in the crash, quoting police.

It was not immediately clear how many people in total were aboard the bus.

Authorities said their initial theory was that the crash was caused by speeding.

The driver "was unable" to control the bus, added Benavides. 

The injured have been taken to a nearby hospital, and some are in "serious" condition, the officer added.