A beluga whale that strayed into the River Seine has died during an attempt to return it to the sea off northern France, authorities said.

Rescue divers overnight guided the whale into a net before a crane lifted it from the river in an operation which lasted six hours. It was then transported in a refrigerated truck to the Channel port of Ouistreham.

During the road journey, the already-weak beluga suffered respiratory failure and it was euthanised by vets shortly after arriving in Ouistreham.

"Despite an unprecedented rescue operation, it is with sadness that we announce the cetacean's death," the local Calvados authorities said.

The plan had been to move the malnourished whale to a saltwater basin in the hope it might recover some strength before being transferred to the sea.

But it was a risky venture.

The four-metre male weighed 800kg, well below an adult beluga's typical weight of 1,200kg, and had not fed since it was first spotted in the Seine last week.

Rescuers believe it was sick before it wandered up the river to within 70km of Paris.

The whale's condition deteriorated during the road journey to the coast. It was still alive when it reached Ouistreham but was weak and struggling to breathe.

Marine conservation group Sea Shepherd France, which was involved in the rescue operation, said leaving the whale in the Seine would have condemned it to certain death.

Beluga cannot survive prolonged periods in the warm, freshwater of rivers.

"(The operation) was risky, but essential to give a chance to an otherwise doomed animal," the group added.

In late May, a gravely ill orca swam dozens of miles up the Seine and died of natural causes after attempts to guide it back to sea failed.

Beluga whales typically live in pods in Arctic or sub-Arctic water

According to France's Pelagis Observatory, which specialises in sea mammals, the nearest beluga population is off the Svalbard archipelago, north of Norway, 3,000km from the Seine.

The trapped whale is only the second beluga ever sighted in France. The first was pulled out of the Loire estuary in a fisherman's net in 1948.