MARIANA, Brazil (AFP) - As a tsunami of mud approached their home, seven-year-old Thiago Santos clutched his grandmother in his arms and said, "Jesus help me".

The little boy's name is on the list of those who disappeared when a swell of mud and mining sludge smashed into the village of Bento Rodrigues in southeast Brazil on Thursday, after a waste reservoir at a nearby iron ore mine collapsed.

"My life is over," said his devastated mother, 28-year-old Geovana Aparecida Rodrigues, her eyes red from crying. "I strived to get ahead for him. I went to work for him, to give him a future. Now there's no point. Why go on?"

Geovana works as a welder near Sao Paulo, 650 kilometres outh of Bento Rodrigues.

Thiago's grandmother took care of him during the week, and he spent weekends with his mom.

The grandmother, 58-year-old Darci Francisca dos Santos, survived the disaster. She was found some 500 metres from her house and taken to the hospital.

But there is still no sign of Thiago.

The boy was afraid of drowning, after a near-fatal accident one day when he chased his ball into a swimming pool - without knowing how to swim.

His mother dove in after him and hauled him out.

"He said to me that day, 'Mommy, I almost died. You saved me.' This time I couldn't save him," Geovana sobbed.

Thiago was home with his grandmother when the tidal wave of ochre mud buried three-quarters of their village.

"When they heard the sound of the water, his grandma took him in her arms in the bedroom. He kept saying, 'Jesus help me, Jesus help me,' until she couldn't hear him anymore. The mud had swallowed him up," Geovana.

She is hanging on to a thread of hope. "To God, nothing is impossible," she said. Then she burst into tears again.

So far, authorities have only declared four people dead in the tragedy. Rescuers are still searching for about 20 others who are missing. But hope is fading.

"We'll work down to the last minute to find them alive. We're not searching for bodies," said Minas Gerais state fire chief Luiz Gualberto Moreira.

Rescuers continued excavating the muck Monday, clearing the way for trained dogs to scour the area.

The wrecked village was otherwise empty. The survivors have been evacuated to a shelter in Mariana, 20 kilometres away.

"I don't want to take away anyone's hope - it might be that we find someone alive. But as time passes, hope is fading," Governor Fernando Pimentel said Sunday.

Marcelo Felicio said he has accepted that he will never see his mother alive again.

He works at Samarco, the mine where the collapse occurred, a joint venture between Australia's BHP Billiton and Brazil's Vale.

He blames himself for not leaving his hometown, which he knew was at risk in case of such a disaster.

He was at the office when the alarm sounded. He rushed to the village, where his entire family lived.

"When I arrived, everything was already covered. I had to go around," Felicio said. "When I finally made it, everyone was desperate for news. They told me my brother, my brother-in-law and my niece were all fine. My wife's family was fine. But they told me, 'Your mom wasn't at your sister's.'"

He learned later that his mother, Maria das Gracas Celestino da Silva, 65, was at his house when the mudslide hit.

"She shut the door and ran. But she was old, and the mud swallowed her," he said.

"If she's alive, it would make me immensely happy. But if God decided to keep her, I hope she's at peace, because she was a great mother. I love her so much."

Then he broke down in tears.