A young whale stranded for five days in a Danish fjord died despite massive attempts to rescue the marine mammal and help it find its way back to sea, veterinarian authorities said.

"A veterinarian who examined it confirmed it had died," said Joachim Engell, a conservationist at the regional natural history museum and a witness at the scene.

The 15-metre-long fin whale, weighing between 20 and 30 tonnes, became stranded on a bank in the Vejle fjord in western Denmark at low tide on Wednesday. It became exhausted after struggling to free itself.

Thousands of people had flocked to see the distressed whale as rescuers made repeated attempts to help it return to the water at high tide.

The whale remained stuck in the same spot for two days, and experts had by Friday decided to allow the whale to "die naturally and in peace".

Fire-fighters were spraying it with water to protect it from the sun in what was assumed to be its last moments.

The whale, however, surprised everyone by suddenly breaking free from its captivity and began swimming again, with observers hailing it as a "miracle" recovery.

Unfortunately, the whale, helped by the tide, began swimming towards the back of the fjord instead of out to sea and, after covering just 1.5 kilometres, was soon trapped again near the port of Vejle in 1.5 metres of water at low tide.

Despite repeated attempts, rescue workers failed to turn around the whale, believed to be three or four years old, and help it back to deeper waters. Fin whales are the second largest living animal after the blue whale, according to the environmental charity WWF. They are listed as endangered by the International Union for the Conservation of Nature.

The one that died on Sunday was the first to get stuck along the Danish coast since 1958.