Hopes of saving a malnourished beluga whale that has swum up the Seine river were receding on Sunday, as rescuers said they were in a race against the clock to find a solution.

The whale was first spotted on Tuesday in the river that runs through Paris to the English Channel. Since Friday it has been between two locks some 70 kilometres (44 miles) north of the French capital.

But leaving it in the warm stagnant water between the lock gates is no longer an option.

"He has to be moved in the coming 24-48 hours, these conditions are not good for him," Sea Shepherd France head Lamya Essemlali told AFP.

Specialists held out "little hope" for the visibly underweight whale as they were "in a race against the clock" to save the creature, Essemlali said.

"We are all doubtful about its own ability to return to the sea," she said. "Even if we 'drove' it with a boat, that would be extremely dangerous, if not impossible".

Before swimming between the two locks, "he had the tendency to be heading toward Paris. It would be catastrophic if he reached there," Essemlali said.

However, "the euthanasia option has been ruled out for the moment, because at this stage it would be premature", she said.

The whale still has "energy ... turns its head, reacts to stimuli", she said after a meeting of experts and French officials.

Although rescuers have tried feeding it frozen herring and then live trout, the animal was refusing the food.

"His lack of appetite is surely a symptom of something else... an illness. He is malnourished and this dates back weeks, if not months. He was no longer eating at sea," Essemlali said.

On Saturday veterinarians had administered "vitaminsand products to stimulate its appetite," said a statement on Sunday by the police in Normandy's Eure department, which is overseeing the rescue effort.

The small spots that were reported on its pale skin on Saturday were due to the fresh water, it said.

Another option under consideration would be to take it out of the water, give it vitamins, check the cause of the illness and eventually ship it out to sea once it regains its strength.

Another is "to let it end its life peacefully, like someone who is very ill and who does not have much chance to live", said Isabelle Dorliat-Pouzet, a senior police official in Evreux.

Belugas are normally found only in cold Arctic waters, and while they migrate south in the autumn to feed as ice forms, they rarely venture so far.

An adult can reach up to four metres (13 feet) in length.

According to France's Pelagis Observatory, specialised in sea mammals, the nearest beluga population is off the Svalbard archipelago, north of Norway, 3,000 kilometres (1,800 miles) from the Seine.

It is only the second recorded sighting of a beluga in a French river since 1948, when a fisherman in the estuary of the Loire river found one in his nets.

The sighting comes just a few months after a killer whale -- also known as an orca, but technically part of the dolphin family -- became stranded in the Seine and was later found dead between Le Havre and Rouen in late May.

Officials said they had also discovered a bullet lodged in the base of its skull -- though it was far from clear that the wound played a role in its death.