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

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

The bus, which was taking children home after school late Monday afternoon, caught fire just 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 official said, adding that the kindergarten's executive manager, Chu Lili, is under police control.

The document 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.

Burning straw has caused a grey haze to envelop major wheat-growing regions this week, including Henan, Anhui and Shandong provinces.

Local governments forbid straw burning, but farmers continue to burn straw as there is no profit in recycling it and leaving it on farmland will affect the next season's crop growth.