A family of five including three children all drowned in underwater quicksand while they were collecting sand from a river in Thailand on Monday.

Wiriya Khadchan, 45, his daughter Nucharin Khadchan, 23, and his grandchildren Waranya Pimthong, 14, Yaya Thongkhamsan, 11, and Foam Nucharam, 8, were collecting the earth to do building work at their home in Buriram province.

However, one of the children is believed to have disappeared underwater when they sank into the soft sand on the river bed.

Tragically, when the other four ran over to help, they were also pulled into the quagmire and submerged by the water that flowed above their heads.

Police said that when the family could not be reached and failed to return home by noon, Wiriya's wife Bang-on Surit, 44, became worried and called the emergency services to search for them.

They scoured the banks of the river before finally finding a tractor filled with sand next to the water at 3 pm. There was a backpack, children's toys, and sandals, but no sign of the missing family members.

Divers plunged into the stream to continue the search. However, Bang-on's hopes for the family's safe return were crushed when they were finally found three hours later - all five of them stuck in a pit of quicksand on the riverbed.

An emotional Bang-on, who fainted when her relatives were pulled from the river, said: 'My husband said he would gather sand to lay the tiles at our house. He brought along our daughter and grandchildren so they could play in the water, but after that, I couldn't contact him again so I called the police and went to search. I didn't think they would all die like that.

'I don't know how it happened. My husband was experienced with the river. I cannot believe my whole family died in that way. I feel so sorry for them when I think about them dying in the sand, they would have been so scared.

'I have lost three generations of our family. I will carry this pain for the rest of my life.'

Police Lieutenant Colonel Thanormjit Kanyayon of Chumphonburi Police Station said officers had not found any witnesses to the tragedy. However, they believe the whole family died when they tried to help each other out of the quicksand.

The officer said: 'We believe one person had become trapped in the quicksand and drowned, with the others also stepping in the quagmire when they rushed over to help.

'There is nothing suspicious and we are not looking for anyone else involved. We will confirm the cause of death from the police doctors.'

Officials have retrieved the villagers' bodies for post-mortem examinations. They will then be cremated in a traditional Buddhist ceremony at the village temple.

Underwater quicksand can be formed from the riverbed being disrupted by water being forced up from a natural spring or aquifer. This creates a loose and saturated mixture that acts like quicksand, losing its strength and being unable to support weight.

The sludge can also be formed from above, when the bed is disrupted by the force of new water in the river, such as from flooding or heavy rain. The sand particles on the river bed lose their cohesion and become fluidlike.