PAKSONG, Laos — Petchinda Chantamart first heard what sounded like a bomb going off a few miles away. Then came a curious noise, like a strong wind.

She knew instinctively what it meant: One of the new dams under construction near her village in southern Laos had given way. She began banging on her neighbors’ doors, she recounted, urging them to flee to higher ground.

“The water is coming!’’ shouted Chantamart.

Within a half-hour, the water in her village, Xay Done Khong, was more than 30 feet deep and rising.

Chantamart, 35, and many of her neighbors escaped the deadly flood. But others were not so lucky when an auxiliary dam, part of the billion-dollar Xe-Pian Xe-Namnoy hydroelectric project, failed Monday evening amid heavy rains, sending more than 170 billion cubic feet of water rushing downstream.

At a news conference Wednesday, Prime Minister Thongloun Sisoulith said 131 people were still missing and more than 3,000 were homeless. Many had been rescued from rooftops and trees after villages and farmland were flooded.

At least 26 people have been reported killed.

“A second step for us will be to recover and identify the deceased, but for now, we hurry to find those who are still alive in the area,’’ Bounhom Phommasane, the governor of the district of Sanamxay, told The Vientiane Times.

Chantamart said hundreds of people from her village had escaped, but that 15 people were still missing, nine of them children. She had been unable to reach their homes Monday because the floodwaters had already climbed too high.

“I’m very worried about them, from the bottom of my heart,’’ she said.

After she and hundreds of others scrambled to higher ground Monday, soldiers and local officials moved them to the town of Paksong, west of the dam site, to take refuge in an empty warehouse normally used to store coffee.

Video posted by the Thai News Agency showed vast quantities of water cascading over what appeared to be the diminished structure of the dam, known as Saddle Dam D.

The official Lao News Agency reported that the dam had collapsed. The main builder of the hydropower project, SK Engineering & Construction of South Korea, said it would investigate whether the dam had collapsed or overflowed because of heavy rains.

International Rivers, an advocacy group that has opposed the rapid growth of hydropower dams in Laos, said in a statement posted online that the auxiliary dam collapsed as flooding from heavy monsoon rains caused it to overflow Monday night.

The group, which seeks to protect rivers around the world, said the disaster showed that many dams were not designed to handle extreme weather events, such as the rains Monday.

“Unpredictable and extreme weather events are becoming more frequent due to climate change, posing grave safety concerns to millions who live downstream of dams,’’ International Rivers said.

People living below the dam had only a few hours’ warning to evacuate before it failed, according to the group.

“Communities were not given sufficient advanced warning to ensure their safety and that of their families,’’ the statement said.

Seven villages in Sanamxay, which is in Attapeu province, were flooded and more than 6,000 people were displaced by the dam’s failure, officials said.

The Xe-Pian Xe-Namnoy is one of 70 hydropower plants that are planned, underway or have been built in Laos, most of them owned and operated by private companies, International Rivers said.