Police are searching for a nine-year-old boy who was snatched by a crocodile as he was swimming in the mouth of a river in northern Australia.

Northern Territory police say the boy was swimming with a group of people at Port Bradshaw when he was grabbed by the crocodile.

Nearby people attempted to kill the crocodile with spears, but it then dragged the boy farther into the water.

Police planned to continue searching but expressed pessimism about the chances of finding the boy alive.

Two weeks ago, another crocodile snatched a seven-year-old girl who was swimming at a Northern Territory waterhole.

The crocodile was fatally shot the next day and the girl's remains were found inside it.