Environmentalists will try to move a dangerously thin beluga whale that strayed into the River Seine last week in a final attempt to save its life.

The mammal, measuring 4m (13ft), will be transported to a saltwater river basin for a “period of care” by medics who believe the animal is sick, said Lamya Essemlali, president of Sea Shepherd France.

If the whale responds to treatment, it may finally be released back to the open sea.

While it was not immediately clear how environmentalists will transfer the whale to the river basin, French media said they might use a barge and then a truck.

Conservationists are hoping to spare the whale the fate of an orca that strayed into, and then died, in the Seine in May.

The marine conservation group has described plans to move the beluga – which was first spotted last week in the Seine, worryingly far from its Arctic habitat – as an “enormous operation”.

While the move “has a risk factor because it’s stressful” for the animal, it cannot survive much longer in the Seine’s fresh water, the group said.

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Because of the extreme heat in France’s Eure region, rescuers are expected to wait until late afternoon before moving the creature, which weighs about 800kg and which French media say may be transported on a barge and covered in wet towels to keep it cool.

The authorities remain hopeful the whale may survive after it responded to a cocktail of antibiotics and vitamins in the last few days and visibly “rubbed itself on the lock’s wall and got rid of patches that had appeared on its back”.

Ms Essemlali said that the medical surveillance at the saltwater river basin will help establish whether the mammal “is suffering from something we can help it with, or from an incurable illness”.

Drone footage shot by French fire services last week showed the whale gently meandering into a stretch of the river’s light green waters between Paris and the Normandy city of Rouen that is far inland from the sea.

Conservationists have tried unsuccessfully since Friday to feed fish to the creature, with Sea Shepherd fearing the whale is slowly starving in the waterway.

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