At least six children have been killed in a school bus crash in Chattanooga in Tennessee, according to police.

The bus was carrying 35 children home from Woodmore Elementary School when it crashed in to a tree, and more than 20 of them were injured.

Officials said five children were found dead on the bus, and another has died in hospital.

According to reports the driver of the bus has now been charged in connection with the incident.

Melydia Clewell, a spokeswoman for the Hamilton County District Attorney's Office, said the number of fatalities may rise as there are two or three children in hospital "who could go either way".

The crash happened at about 4.00pm local time (9.00pm Irish time), and the bus was the only vehicle involved in the accident.

Pictures from the scene show the school bus turned on its side, and Ms Clewell said the impact left it nearly severed in half.

Police said they would not identify which students died, or their ages, until their parents are notified.

The children on board were students in classes from kindergarten to fifth grade, which would make them between five and 11-years-old.

Chattanooga Police Chief Fred Fletcher said: "Our hearts go out, as well as the hearts of all these people behind me, to the families, the neighborhood, the school, for all the people involved in this, we assure you we are doing everything we can."

Tennessee state Governor Bill Haslam described the crash as a "tragic event" and offered assistance.

Police are interviewing the bus driver to try to establish what happened, and prosecutors have requested a blood sample from him.

At a news conference, officers said speed was being investigated "very, very strongly" as a contributing factor in the crash - and the possibility of alcohol or drug use as a factor is also being examined.