Four people aged four to 18 were killed Monday when a vehicle plowed into a building housing an after-school camp in the US state of Illinois, police said, adding they were investigating the crash.

Several others were injured and transported to hospital, Illinois State Police said in a statement, while the driver of the vehicle was uninjured and taken to hospital for evaluation.

"At this time, we can confirm four fatalities believed to be between the ages of 4-18 years. Three individuals were struck outside the building and one inside," the statement read.

"Several additional individuals have been transferred to local area hospitals by ambulance and one by a life flight helicopter."

Police responded to the crash in Chatham, a small town around 10 miles south of state capital Springfield, at around 3:20 pm (2020 GMT) on Monday.

Illinois Governor JB Pritzker shared a statement on X saying he was "horrified and deeply saddened" by the incident.

"Parents said goodbye to their kids this morning not knowing it would be the last time. My heart is heavy for these families and the unimaginable grief they're experiencing -- something that no parent should have to endure," he wrote.

The after-school center, YNOT After School Camp, says on its website that it specializes in providing outdoor activities for children.