Tapan Kumar Dey, a former forest conservator who was overseeing the rescue operation, said the elephant died despite the "highest efforts" to save him.

"This is very sad. We tried our best to save it," he said.

The cause of the death of the elephant named "Bangabahadur," or Hero of Bengal, was not immediately clear.

The elephant, tired and weak from its struggle, had been tranquilised earlier in an attempt to steer it from the swamp and to bring it closer to a road so it could be transported to an elephant safari park.

The elephant appeared to be fine on Sunday, Mr Dey said, but was likely to have become dehydrated after being stuck in the swamp for days.

Monsoon-triggered flooding had carried the elephant from upstream India before he became trapped in a swamp in Bangladesh's Jamalpur district three weeks ago.

Authorities had planned to rescue him and move him to the safari park near the capital Dhaka.

Earlier, Indian wildlife authorities had abandoned plans to take the elephant back because it was unlikely that he would have been welcomed back to his herd in the hilly forests of the remote north-eastern state of Assam.

Heavy downpours have flooded vast areas of eastern India since monsoon rains began in June.