Authorities in the US state confirmed more than 20 children who attended the Camp Mystic summer camp were still missing as a result of flash floods along the Guadalupe River.

The death toll from sudden floods in the Texas Hill Country rose to 24 Friday night, officials said, as the search for more than 20 people missing from a girls summer camp continued through the night.

Rescuers were desperately searching for at least 20 girls missing from a riverside summer camp, officials said Friday, after torrential rains caused a "catastrophic" flash flood that killed at least 24 people as it swept through south-central Texas.