Two helicopters are tackling an "aggressive" fire that has been burning in the middle of the Waitaki River near the South Island town of Kurow.

Fire senior communicator Mau Barbara said the fire on a small island in the river was reported shortly before two this afternoon.

It was located near the Kurow-Duntroon Road some nine kilometres from Kurow town.

Despite calling the fire aggressive, Mr Barbara said the vegetation fire was not posing any risk to people or residents.