They say the animal is currently near a lock at Vernon, about 70km (43 miles) north-west of Paris.

Members of the public are being urged to keep away from the whale that appears to be underweight.

Experts are puzzled as to why the whale has strayed so far south from its natural habitat.

“The challenge now will be to help feed it, and try to accompany it towards the ocean,” Lamya Essemlali, the head of environmental group Sea Shepherd France, was quoted as saying by the Reuters news agency.

She said taking the animal out of the water was out of the question as it was too risky.

Pictures have emerged showing the whale swimming slowly in the river.

The officials have not reported on the size of the animal, but an adult beluga can grow to more than four metres (13ft) in length.

In May, a killer whale was found dead after swimming up the River Seine in Normandy. A plan to guide the four-metre male orca back to the sea using sound stimuli failed, and experts later concluded it was seriously ill.

In 2019, a dead whale was found in the River Thames near Gravesend, UK officials said.

This came just weeks after a humpback whale seen swimming in the same stretch of water had died. It was thought to have found its way into the Thames because of a navigational error, possibly during high tides.