The crash occurred around 11.30am when the bus, for unknown reasons, crossed into oncoming lanes on state Route 24 in front of the truck, which was headed in the opposite direction, a state police spokesman told a news briefing.

The two vehicles became engulfed in flames on impact, she said. News images from the scene showed the charred, burned-out wreckage of the bus.

All three children, who were the only passengers aboard the bus, perished in the crash, along with the drivers of both vehicles. All were pronounced dead at the scene, police said.

ABC News, citing the Schuyler County coroner, reported the three children were preschoolers. The Schuyler-Industry school district said in a Facebook post that classes would be cancelled on Tuesday and Wednesday out of respect for grieving families.

The wreck occurred near the town of Rushville, located in rural west-central Illinois, about 96.5km northwest of Springfield, the state capital.

“There’s not really words at this time that can be said to anybody,” Sheriff Bill Redshaw told reporters at a local fire station. “Schuyler County is a small county, a small community. Rushville, Illinois, it’s a close-knit family.”

The cause of the crash, including questions about whether alcohol or drugs were a factor, remained under investigation, police said.