A malnourished beluga whale that has swum up France's River Seine is no longer progressing but is still alive, environmental group Sea Shepherd said Monday.

Hopes are fading to save the animal, which was first spotted on Tuesday in the river that runs through Paris to the English Channel.

"It is alert but not eating," Sea Shepherd France president Lamya Essemlali told AFP in a text message.

There was, however, "no worsening of its condition", she said.

Since Friday the whale has been between two locks some 70 kilometres (44 miles) north of the French capital.

Rescuers are considering last-ditch efforts to extract the animal from the Seine as the river's warm water is harming its health.

One alternative would be to open the locks in the hope that the beluga will swim towards the English Channel, authorities said.

Opening the locks would harbour the risk of the whale moving further upriver towards Paris, which would be even more disastrous.

Several attempts to feed the whale have failed in the past days.

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

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 from the Seine.