An ailing killer whale adrift in France's River Seine will be euthanised after a plan to guide it back to sea failed and scientists concluded it was in agonizing pain and terminally ill, the local prefecture said on Sunday.

The 13-foot 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, before it swam dozens of miles upstream to reach west of the city of Rouen.

Following a meeting with national and international scientists, including marine mammal specialists, the local prefecture tried on Saturday to guide the whale back to the sea with a drone while emitting orca sounds.

But the whale responded "erratically" and "incoherently" to the sound stimuli, the prefecture of the Seine maritime area said in a statement. Sound recording of the whale captured what they said was akin to distress calls by the animal.

"The attempt to bring back the whale to sea having failed, and to prevent adding to it stress levels, a decision was made to stop the intervention in the evening," it said.

Scientists reviewed pictures and data from the intervention and concluded that the animal was suffering from mucormycosis, or black fungus, a disease seen in whales in North America but that had not been observed in Europe yet.

After infecting the skin of weakened animals, the disease can spread to the heart, lungs, and brain, which explained the whale's disorientated behavior, the prefecture said. It added that the disease seemed very advanced in this case and was probably causing the whale major suffering.

"In these conditions, the expert group concluded unanimously that the only possible solution was to euthanise the animal to end his suffering and, also, to carry out advanced analysis of the disease it is carrying," the prefecture added.

Experts from the French office for biodiversity (OFB) as well as the Sea Shepherd NGO were among the specialists involved in the failed rescue operation.