BANGKOK -- A hydroelectric dam collapsed in southeastern Laos, leaving an unknown number of people dead and hundreds missing, state media said Tuesday. Rescue efforts were underway as top government officials rushed to the site and public appeals were launched for aid.

The official Lao news agency KPL said the Xepian-Xe Nam Noy hydropower dam in Attapeu province collapsed Monday evening, releasing large amounts of water that swept away houses and made more than 6,600 people homeless.

The dam was constructed by a joint venture led by South Korean companies, with Thai and Lao partners. The project was still under construction, KPL reported. It described the portion that collapsed as a "saddle dam," which is an auxiliary dam used to hold water beyond what is held by the main dam.

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Prime Minister Thongloun Sisoulith "suspended the planned monthly meeting of the government for August and led his Cabinet members and other senior officials to Sanamxay (district) to monitor rescue and relief efforts being made for flood victims," KPL said. Many areas of Laos have recently been hit by floods from heavy seasonal rains.

Provincial authorities issued a call for emergency aid - clothing, food, drinking water, medicine, cash and other items - from the "party, government organizations, business community, officials, police and military forces and people of all strata."

Laos, one of the poorest countries in Asia, has transitioned from communism to a market economy but remains a single-party state where freedoms are limited. There is virtually no freedom of the press, and foreign reporters who visit operate under tight restrictions, limiting the flow of information.

Electricity from several hydroelectric dams provides a large share of Laos' export earnings, with Thailand being a major buyer.

KPL said the Xepian-Xe Nam Noy project cost an estimated $1.02 billion.

According to the website of the company that built and runs the dam, it is majority-owned by two South Korean companies, SK Engineering and Construction and Korea Western Power. Most of the financing for the project came from Thai lenders. The Ratchaburi Electricity Generating Holding Public Co. Ltd of Thailand holds a 25 percent stake and the Lao Holding State Enterprise holds 24 percent.

The dam was built to divert the Houay Makchanh, Xe-Namnoy and Xe-Pian rivers into reservoirs that feed into a 410-megawatt power plant designed to generate 1,879 gigawatts of power a year, with 90 percent of the power being exported to Thailand and the remaining 10 percent used locally. The project is a 27-year concession and was due to begin operating in 2019, a year later than originally planned.

The project is on a volcanic plateau divided by a river gorge and the catchment area accounts for 17 percent of the Mekong river's annual flow.

According to assessment documents, about 30 villages were affected by the project with more than 2,000 people in eight villages resettled. Roughly 10,000 people live in the affected area, with most belonging to ethnic minorities.

The project was supposed to be a cash cow for SK E&C for years. The company is part of the SK Group, one of South Korea's top three conglomerates. Its units include SK Hynix, the world's second-largest chipmaker, and SK Telecom, South Korea's largest telecom carrier.

Thailand was to receive a stable supply of electricity while Laos was supposed to receive taxes, royalties and other income estimated at $33 billion per year.

South Korea's Yonhap News agency said SK E&C has dispatched its president to Laos and has set up an emergency team in Seoul. SK E&C did not respond to several calls seeking comment.

Yonhap said SK E&C was still trying to find out whether water had overflowed the dam or if the dam had collapsed.

Yonhap quoted an unidentified SK E&C official as saying rain in the area was three times more than the usual amount, and that one of five auxiliary dams had overflowed.

It said SK E&C is focusing on the rescue operation, using boats and other equipment to search for missing people.