The kindergarten minibus, only equipped with nine seats, was transporting 62 children and two adults [Reuters]

An overloaded school minibus has crashed head-on with a truck in rural western China, killing at least 18 kindergarten children on their way to class, officials said.

Two adults, the driver of the minibus and a teacher at the kindergarten, also died in Wednesday's accident, said Fan, the director of the emergency office of the Gansu provincial work safety bureau.

The bus had nine seats, but Fan said it was jammed with 62 children and two adults when it collided with the truck in China's Gansu province, leaving the orange school vehicle crumpled and twisted.

Authorities blamed the overloading for the accident, the official Xinhua News Agency said.

Four children and the bus driver died at the scene, said Fan.

Gao Shaobo, head of traffic police in Zhengning county, where the school is located, later said that 20 people had died and 44 were still hospitalised - two in critical condition and 12 with serious injuries.

Xinhua reported that the truck was loaded with coal, but Gao told state broadcaster CCTV that it was used to transport stones but empty at the time of the accident.

The bus was on its way to the Little Doctor Kindergarten on the outskirts of Qingyang city after picking up the children - aged five and six - when the accident happened, Gao said.

"The van was driving on the wrong side of the street. Both the truck and the van were going at high speeds at the time," he said.

The two people in the truck were not injured, and the driver has been detained, Gao said.

Oercrowding on school buses is common in China, and accidents happen frequently because of poorly maintained vehicles and poor driving habits.

The latest acident appeared to be one of the worst in China in recent years.