At least 20 people have been killed after flash floods and heavy monsoon rains battered Laos, according to a United Nations official in the country.

Many have been left without access to clean water, roads have been washed away and crops damaged.

Rains along with typhoons that passed Laos in June and July have affected about 116,000 people in seven of 17 provinces and damaged 14,000 hectares of rice, the United Nations said.

"We've had near-misses from typhoons in the north of the country and received the rains that came with them," Glenn Dodge, head of the UN Resident Coordinator's office in Laos, said.

The UN warned last year that the country of 6.4 million could face more natural disasters due to climate change and needed to be better prepared.

Isolated for decades, Laos has become one of Asia's fastest-growing economies thanks to a mining and hydro-power boom.

The landlocked communist country is constructing a €2.6 billion Chinese-backed hydropower dam - the first of 11 planned for the lower Mekong river - despite concerns raised by environmentalists.