Rescuers were searching for the bodies of two boys presumed to have drowned at Blue Dam in Fleurhoff on Sunday, Johannesburg emergency services said.

Spokesperson Percy Morokane said three boys, aged six, eight and nine, had gone to the dam at around 11am.

The youngest boy jumped into the water and when he began submerging, the eight-year-old tried to save him.

"But both boys, who were best friends, went under," said Morokane.

"The third child saw bubbles on the water surface and ran about two kilometres to Meadowlands to inform their parents."

Both boys were presumed dead. Their bodies had not yet been recovered.