Vietnam on Tuesday mobilised soldiers and engineering experts to try free a 10-year-old boy trapped in a buried hollow concrete pillar on a construction site.

Rescuers hope to raise the pillar from its 35-metre-deep hole and cut the boy, named Thai Ly Hao Nam, out - though after three days trapped inside, it is not clear whether he is still alive.

The youngster fell into the 25-centimetre wide shaft of the pillar, sunk as part of a new bridge in Mekong delta province of Dong Thap, on Saturday, apparently while looking for scrap metal.

A wider 19-metre-long metal pipe has been lowered around the concrete tube in which Nam is trapped to allow them to remove mud from around the pillar and try to lift it out.

Rescuers were softening and removing mud and water to reduce pressure around the pillar before bringing it up - most likely later on Tuesday evening.

Around 100 soldiers and professional equipment have been deployed at the site, which has been sealed off from the public while the rescue effort goes on.

"The boy has been trapped inside the small pillar for four days, with assumption of multiple injuries. Hope for him to be alive is limited," said Doan Tan Buu, deputy chairman of the Dong Thap provincial people's committee.

Two days of rescue efforts have so far had no success.

"We had tried our best to rescue the boy using on-the-spot measures and equipment," said Buu.

"We later had to report and asked for help from national levels and people with expertise," Buu was quoted by state-controlled Tuoi Tre newspaper.