(CNN) An estimated 25,000 people are being evacuated from a northern Cambodian province following extensive flooding due to the collapse of a massive hydropower dam in neighboring Laos.

Authorities in Stung Treng province are attempting to evacuate towns and villages downstream from the dam, which has caused waters to rise above 12 meters (39 feet) in places, according to Cambodia's state news agency.

The agency quoted a spokesman for the Cambodian Ministry of Water Resources and Meteorology who said there was no sign of waters receding as of Thursday afternoon.

The collapse of the hydropower dam has already left at least 26 people dead and another 6,000 homeless in southern Laos, the worst disaster faced by the small Southeast Asian country in decades, according to Laotian Prime Minister Thongloun Sisoulith.

"The strong tide of water from the broken dam immediately flowed through the community and washed away houses and villagers; many people were unable to move to the highland," Sisoulith said at a news conference Wednesday after visiting the affected areas.

People use a makeshift ferry to cross the swollen Xe Khong Rver due to flash flooding Wednesday.

The Xepian Xe Nam Noy Dam, a billion-dollar hydropower project part financed by South Korea, was still under construction when the breach occurred around 8 p.m. Monday in Laos' sparsely populated Attapeu province.

The dam's collapse occurred without warning and inundated entire villages with more than 5 billion cubic meters of water.

Bounhom Phommasan, Attapeu province's district governor, said authorities have rescued 2,851 from the floods but more than 3,000 are awaiting rescue, according to Laos state media.

At least 26 bodies have been retrieved from the muddy floodwaters. Rescuers are still searching for 131, according to the Prime Minister.

Up to 11,777 people were affected by the dam collapse, with up to 6,000 displaced, according to figures from the United Nations. Aerial images showed towns swallowed by water, with residents waiting on rooftops, with only a few possessions in hand.

People are stranded on rooftops Tuesday after a dam under construction collapsed in Laos.

The dam was built on a tributary of the Mekong River, the longest river in Southeast Asia. The Mekong, which runs from the Tibetan Plateau through China, Myanmar, Thailand, Laos, Cambodia and Vietnam, is an important regional resource, providing fish stocks and electricity -- in the form of hydropower -- to the people who live along it.

The flooding in southern Laos was expected to cause water levels to rise downstream, according to the Mekong River Commission, the joint Lao-Cambodian-Vietnamese organization that manages the shared resources of the Mekong.

There is no news yet on what the damage or the death toll might look like in Cambodia, downriver of the shattered dam.

Sisoulith said an international rescue effort is underway, and authorities are working to relocate those who live downstream of the areas already flooded, which could be inundated in coming days.

Criticism of authorities

Some have criticized Laotian authorities for improper oversight and a lack of communication in the disaster's aftermath.

International Rivers, a US-based nongovernmental organization that has campaigned on behalf of residents who have opposed the dam's construction since 2013, said the collapse has exposed the need for greater regulation for dam construction.

"Many dams in operation or planned are not designed to be able to cope with extreme weather events," the group said in a statement.

"Communities were not given sufficient advanced warning to ensure their safety and that of their families. This event raises major questions about dam standards and dam safety in Laos, including their appropriateness to deal with weather conditions and risks."

Sebastien Perret, a member of Vientiane Rescue 1623 -- an independent aid group that helps with disaster response throughout Southeast Asia -- told CNN in Attapeu that the water appears to be receding slowly in southern Laos, but the weather changes hour to hour.

The most affected areas are remote, making aid efforts even more challenging, Perret said, adding it took his team 18 hours to get to the flooded areas of Attapeu, where Vientiane Rescue 1623 is helping authorities recover bodies and rescue survivors.

"This kind of disaster doesn't happen in this country very often, so we need more boats, we need more helicopters," he said.

Inquiry to be launched

Daovong Phonkeo, director-general of Laos' Department of Energy Policy and Planning, told CNN he believes the rescue effort has been well-organized.

The department was in charge of writing the safety standards for the dam, and Phonkeo said it will launch an inquiry to see if private companies building the dam abided by those regulations.

Laotian soldiers carry rescue equipment Wednesday in Attapeu province.

The dam was one of many being built in the Mekong Delta to bring renewable electricity to one of the world's most underdeveloped regions. Two of Attapeu province's most important industries are hydropower energy and electricity generation, according to the Laotian government.

Critics of the project said dams on the Mekong do not outweigh its impact on the environment and local ecosystems.