Following a meeting with national and international scientists, including marine mammal specialists, the local prefecture said it would monitor the killer whale from a distance with a drone while emitting orca communications in an attempt to guide it out of the river.

"The use of these non-invasive methods, from several hundred metres distance, will make it possible to avoid using ships in the immediate proximity of the animal, which could aggravate its stress and endanger its survival," the Seine-Maritime prefecture said in a statement posted on Twitter.

The whale, whose health is deteriorating in fresh water, strayed from the ocean earlier this month, and is at risk of dying.

The four-metre orca, identified as a male, was first spotted at the mouth of the Seine on May 16 between the port of Le Havre and the town of Honfleur in Normandy on France's north coast, before it travelled dozens of kilometres upstream to reach west of the city of Rouen.

Several French media outlets showed footage of the killer whale in the river, its dorsal fin sticking out of the water and its distinctive black and white colouring showing as it comes up for air.

Australian Associated Press