US race car driver killed by rival

A US race car driver has been killed after he was run down by a competitor following an on-track incident.

WARNING: The above video contains graphic content and may distress some viewers.

Former NASCAR racing champion Tony Stewart is being investigated by police after an incident on the race track at the Canandaigua Motorsports Park on Saturday night.

Ontario County Sheriff Philip Povero said his department's investigation is not criminal and that Stewart was "fully cooperative."

Povero would not identify the driver, but said he was pronounced dead Saturday night at a hospital in Canandaigua.

However, video has emerged of fellow driver Kevin Ward Jr. walking out into oncoming traffic before being hit by Stewart.

The incident happened during a Empire Super Sprints race which was being held on the eve of a NASCAR race at Watkins Glen International where Stewart was scheduled to compete on Sunday.

Stewart appeared to hit Ward's car during the race, forcing the vehicle to spin out of the contest and bring out the yellow flags.

An irate Ward then emerged from his car and braves the oncoming traffic to remonstrate with Stewart as he rounds the bend.

But in horrific scenes, Stewart ploughs straight into the vulnerable Ward, sending him flying through the air.

Witnesses said three-time NASCAR Sprint Cup Series champion Stewart did not appear to have been injured.

The rest of the Empire Super Sprints races were cancelled after the incident.