A sudden flood tore through villages in north-central Nepal early Sunday.

The flash flood on the Budhi Gandaki river swept away at least 40 houses across five settlements in Gorkha district, Moneycontrol reported. Four people were reported missing. Bridges and other infrastructure were damaged.

The flooding began around 4 a.m. in Chumnubri Rural Municipality, reportedly after a landslide blocked a stream upstream. Officials suspect the temporary dam then burst, sending a surge of water, mud and debris downstream.

Nepal's National Disaster Risk Reduction and Management Authority evacuated villagers and told others to move to safer areas, the Associated Press reported via ABC News. People were warned to stay alert until late Sunday. There were no immediate reports of deaths.

Terrified onlookers filmed from higher ground as the water swept away buildings, a bridge and homes, Al Jazeera reported.

Nepal is especially at risk from floods and landslides. Its steep Himalayan valleys funnel water quickly, and monsoon rains loosen hillsides. Melting glaciers are also creating larger and less stable lakes high in the mountains.

In late August, a collapsing glacier high on Langtang Lirung set off devastating flash floods that killed more than 1,300 people in Nepal and left thousands missing, Gizmodo reported in September. Scientists warned that floods like that may become more common as the Himalayas warm.

Rescue teams were trying to reach the affected areas. The remote terrain makes rescue work slow.

The cause of Sunday's flood was still being investigated. Officials said they would update the number of missing as teams reached the area.

What we know: a flash flood swept away homes along the Budhi Gandaki river.
What we don't: how many people are missing or what triggered the flood.