(Reuters) - About 20,000 gallons (76,000 liters) of crude oil belonging to Exxon Mobil Corp was released after a Canadian National Railway Co freight train derailed on Friday at Plainfield, Illinois.

The cause of the derailment was unknown, a filing with the Illinois Emergency Management Agency said on Saturday.

Twenty cars of the south-bound Canadian National train carrying crude oil were involved in the derailment, while three of the cars leaked, CN spokesman Patrick Waldron said.

There were no injuries, fire or threat to public safety, Waldron said, adding the track has been repaired and trains have resumed operating through the area.

Environmental clean-up work is underway at the site of derailment and the investigation into the cause of the accident would continue into this week, he said.