A 10-year-old Vietnamese boy trapped in a buried hollow concrete pillar at a construction site for four days was declared dead on Wednesday.

Rescuers in Dong Thap province in the Mekong Delta had been trying to raise the pillar from its 35-metre-deep (115-feet) hole and cut out young Thai Ly Hao Nam.

The boy fell into the 25-centimetre-wide (12-inch) shaft of the pillar -- sunk as part of a new bridge -- on Saturday, apparently while looking for scrap metal.

Doan Tan Buu, deputy chief of the province, said on Wednesday that the boy had died.

"He has been trapped in a hollow pillar very deep down... with multiple injuries and not enough oxygen for a very long time," he told journalists at the construction site.

"We had prioritised the rescue of the boy. However, the conditions mean it is impossible the boy has survived," he added.

Buu said the announcement of the boy's death was made after consulting with medical experts.

Rescuers are determined to bring his body up as soon as possible for burial, but Buu admitted it was a "very difficult task".

Vietnam on Tuesday mobilised hundreds of soldiers and engineering experts to try to rescue Nam.

A 19-metre metal pipe was lowered around the concrete tube in which the boy was trapped to allow them to remove mud and try to lift it out.

Rescuers had softened and tried to remove mud and water to reduce pressure around the pillar, but days of effort had failed to pay off.