Authorities in Central China's Henan province said the school bus that caught fire on Monday had become stuck on a pile of straw before it was engulfed in flames that killed four children on board.

The family of died children were painfully sad. [Photo / Chinanews.com ]

The bus driver could not move the vehicle after it got stuck on a 70-cm-high pile of straw on a lane in Zhangyi village of Puyang, the city government's information office said in a statement on Wednesday.

The bus, which was taking children home after school late Monday afternoon, caught fire as the driver was trying to clear the road of straw, it said.

Investigators said the overheated catalytic converter and muffler set the straw on fire.

The eight-seat minibus was carrying 13 children at the time of the accident.

Four children died at the scene and three were injured. The bus driver and one of the preschoolers were seriously injured.

The minibus belongs to Doucunji Kindergarten, the statement said, adding that the kindergarten's executive manager, Chu Lili, is under police control.

It said the kindergarten was unlicensed and its school bus, which was purchased in December 2010, had been illegally renovated to run on natural gas.

Puyang's traffic police and education authorities had ordered the bus to halt service in a safety overhaul last year, but the bus continued to operate despite the order.

The safety of school buses in China has become a hot topic after a series of accidents shocked the country.

One of the highest profile accidents happened in November when 19 children and two adults lost their lives in Zhengning county, Gansu province. The bus, designed to carry nine passengers, had been illegally refitted, the seats ripped out and the passengers forced to stand during the journey. The bus was carrying 64 people when it collided with a coal truck.

In another move to strengthen traffic safety, the Ministry of Public Security said on Wednesday that a national traffic safety campaign saw 1,436 bus and truck drivers have their licenses suspended over drug use from March to May.

Of the total, 235 have had their driver's licenses revoked, said a statement from the ministry.

The ministry said that since the campaign was launched in late March, traffic accidents involving buses dropped 21.7 percent over the same period last year.

Drug use by a driver does not automatically constitute a violation of the traffic law on the Chinese mainland, but it does pose a great threat to public security.

Police must cite the anti-drug law to make a detention if a drug-using driver does not violate traffic regulations.

Chinese police launched a nationwide program to test bus drivers for drug use in late April, after a fatal traffic accident in East China claimed 14 lives.

The testing of bus drivers will last until the end of June. Those found to have a history of drug use will be suspended from their jobs, and those who are found addicted will be stripped of their licenses and banned from the business, the statement said.