As if survivors of last week's dam collapse in southern Laos don't have enough to deal with, rescue authorities are warning of another serious danger in the wake of the ensuing flood.

Attapeu was one of the most heavily bombed provinces during the Vietnam war, and unexploded ordinance (UXO) has remained in the ground in unknown quantities for up to half a century.

Far from destroying bombs or landmines that have lain on or in the ground, rain and floodwaters can dislodge and shift UXO buried in soil or mud, and carry it to areas that were previously declared free of landmines.

The dam wall collapse sent water rushing toward villages. ( Facebook: Vientiane Rescue )

Last week's dam wall collapse sent millions of tonnes of mud and water downstream into surrounding villages, burying homes and buildings and inundating roads.

"Families who were once safe from the threat of UXO may no longer be," warns Jerry Redfern, who is producing a documentary about the ongoing threat of unexploded ordinance in Laos.

"Think of that: for years you work the land, knowing that there may be bombs in the soil. Then the land is cleared and you can work without the fear that your hoe might hit a cluster munition.

"And then your land is covered in a layer of mud that might contain more bombs, and you are back where you were: again afraid of the land."

More than 6,000 people were effectively left homeless when the dam waters surged downstream wiping out several villages and washing away homes and buildings.

About 3,000 people are now sheltering in a refugee camp at Sanamxai, the hardest hit district.

Another 2,200 are stranded on land surrounded by water, and unable to reach shelter. The Lao army has used helicopters to drop regular supplies of food and water.

Many of those stranded are waiting till the flood waters recede enough for them to reach drier areas.

Rescue authorities continue to find several bodies each day, many of them children. ( Facebook: Vientiane Rescue )

But emergency authorities say as many as 1,000 more people are missing — possibly alive but stranded in forest areas where helicopter crews can't see them. They say it could be another 2-3 weeks before they can be found or rescued.

Rescue workers have been overwhelmed by the task of finding the dead and missing, and saving survivors, given the ongoing monsoonal rains, which in recent days have caused floods and landslides in other parts of the country.

Using boats and vehicles, they continue to find several bodies a day of people washed away by the floods. Many are children. One body was of a six-month-old baby.

One truck — donated from Vientiane to help in the rescue effort — fell off a bridge at the weekend, because of the water level — killing its driver.

Rescuers try to retrieve the driver of a truck that fell off a bridge because of the high water level. ( Facebook: Vientiane Rescue )

The Lao Red Cross has issued a warning of the increased danger of landmines and "bombies" — the smaller cluster bombs that were dropped in the millions by US aircraft during the Vietnam war.

Jerry Redfern says as the floodwaters recede, that is when the danger of UXO will become clearer.

"If you have a flood like this it could pick up ordinance from a spot that hasn't perhaps been cleared and move it onto your land in that huge wall of mud that has come through.

"And in the reports that have come out in the last day or so, people are seeing mud 3 metres deep in places that has come running down the river, off hillsides etc.

"So there's a real concern … that if you have a major incident like this where a great deal of the topsoil is being moved from places that haven't necessarily been cleared then it could shift a UXO to places that perhaps have been cleared."

Mr Redfern says the added danger is that areas deemed safe have often been cleared only to a depth of about 35-40 centimetres below the surface, meaning heavy flooding can easily uncover mines deeper underground and bring them to the surface.

"I've heard of Lao folks telling stories of, 'we had this area cleared 10 years ago by X group — which of a particular clearance group came through — and then we found more bombs five years later and they came back and they cleared up more cluster munitions after that. And then we're still finding cluster munitions this far on,'" he says.

An estimated two and a half million tons of ordnance rained down on Laos between 1964 and 1973, representing a bombing mission on average every eight minutes for nine years.

Roughly one third — mostly cluster bombs — failed to detonate. Every year people are maimed or killed when they explode, often when they're found by children and occasionally under a house.