The death toll from unprecedented flash floods along the Nepal-Tibet border has risen to 1,252, with another 4,216 people still missing.

The number of deaths and missing people increased sharply from the previous day as authorities continued assessing the disaster.

Nearly 12,000 people have been rescued, while almost all communication towers have been restored with help from domestic and international aid.

About 21,500 security personnel have been deployed to accelerate search-and-rescue efforts, and more than 5,000 injured people are receiving treatment, ABC News has reported.

The flooding was triggered by the failure of part of a glacier and has raised renewed concerns about climate change and the growing risk of extreme weather.