The incident took place near Mahrath village as a speeding Tata Magic 407 crowded with passengers collided head-on with a truck coming from opposite direction, Superintendent of Police (SP) Ashok Kumar Singh said.
Eight persons were killed on the spot, while four others later died in the Jamui hospital where they were admitted.
The eight others injured were being treated in the hospital. The two vehicles collided after the driver of the ill-fated minibus lost control of steering due to certain mechanical problems, the SP said.
The Deputy Superintendent of Police (DSP), Headquarters, Babulal Prasad and Halsi police station's Officer In-Charge Sunil Kumar Jha, besides civil and police officials of Jamui district were camping on the spot to supervise rescue and relief works, he said.
The minibus was carrying over two dozen passengers from Sikandara (Jamui) to Sheikhpura town, Singh said. Four died in Jamui hospital, doctors.