The accident happened in the district of Qilla Saifullah in Baluchistan province.
Deputy District Administrator Mohammad Qasim said rescuers were still trying to transport the bodies to a nearby hospital. There were apparently no survivors, he added.
The exact cause of the crash was not immediately known.
Authorities said the weather was fine at the time of the crash, and that police officers were looking into possible mechanical problems or human error as witnesses said the bus appeared to be speeding when it lost control and fell into a ravine.
Eyewitness Abdul Ali told AP by phone that he was traveling on the same road on motorcycle when he saw a small bus going faster, skidding and falling into a ravine.
Ali said that after the bus fell people from a nearby village alerted police, who rushed to the scene. He said he then joined rescuers who were transporting bodies to the hospital in ambulances.
Qillah Saifullah is located about 200 kilometers (120 miles) north of Quetta, the capital of Baluchistan province.
Pakistan’s President Arif Alvi and other government officials expressed sorrow over the tragic accident, and ordered authorities to make arrangements to deliver the bodies of passengers to their families.
Deadly road accidents such as this one are common in Pakistan due to poor road infrastructure and disregard for traffic laws, as well as poorly maintained vehicles.
In July of last year, a jam-packed bus carrying mostly laborers traveling home for a major holiday rammed into a container truck on a busy highway in central Pakistan, killing at least 33 people.