China has ordered schools nationwide to conduct safety checks on buses after 19 children were killed in a head-on collision between a truck and a massively overloaded kindergarten minibus.

The nine-seater bus, from a kindergarten in the northwestern province of Gansu, was carrying 64 people when the accident happened in Qingyang city on Wednesday morning.

In all, 21 people were killed, the state-run Xinhua news agency reported late Thursday evening, raising the previous toll following the death of a five-year-old boy. The dead included two adults, and 44 others were injured.

Police have detained the head of the kindergarten and the driver of the truck, as well as two deputy county chiefs and the heads of the county's education bureau and traffic department, the official Xinhua news agency said.

An initial investigation showed that the bus had its seats removed to make room for more passengers and was speeding in bad weather, the report quoted Zuo Jianghua, a spokesman for the city government, as saying.

The education ministry responded on Wednesday by ordering education authorities and schools and kindergartens across the country to carry out safety checks on buses.

"(Buses) that have potential safety risks must immediately be taken off the road and be repaired," the ministry said in a statement.

"Vehicles that cannot meet safety requirements are strictly banned from getting onto the road," it said.

Children are often crammed into buses for their journeys to school in China, especially in rural areas, despite strict regulations on such practices.

Parents of students at the kindergarten in Qingyang said school buses had been overloaded for years despite several complaints, according to Xinhua.

In September Chinese police reportedly charged the driver of a school minibus with "seriously overloading" his vehicle after it was stopped with 64 children on board.

The fatal crash on Wednesday was the latest on China's notoriously dangerous roads, where drivers often flout traffic safety laws.

Almost 70,000 people died in road accidents in China in 2010 -- around 190 fatalities a day -- according to police statistics.