The crash took place on Sabzehvar-Neyshabur road at 06:19 hours local time (0249 GMT), the deputy head of a crisis management center in the northeastern Iranian province of Razavi Khorasan, Hamid Hosseini, told IRNA on Sunday.
The other 29 passengers and the bus driver have been injured in the incident, the official added.
He went on to say that seven ambulances, an SOS helicopter, and a car from Neyshabur emergency HQ are present at the scene.
The four wounded who were in critical conditions were transported to hospitals via helicopter, the official underlined.
The overturned bus with 30 passengers, two drivers and a driver helper was on way to Mashahd from Qom in north-center Iran when it lost its control after seeing a car incident happened for a Kia Pride.
The distance between Neyshabur and Mashahd, the capital city of Razavi Khorasan where the holy shrine of Imam Reza (AS) is located, is about 73 km.
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