Recovery efforts are under way in flood-hit Louisiana as the southern state struggles with the aftermath of the worst natural disaster to hit the US since Superstorm Sandy in 2012.

The Federal Emergency Management Agency (FEMA), which oversees the National Flood Insurance Program, has issued more than $US20 million ($26.2 million) in advanced payments to policyholders.

Almost 26,000 policyholders have submitted claims for flood loss.

FEMA has also approved more than $US107 million ($140.3 million) in grants to help survivors with essential home repairs and other pressing disaster-related needs.

A spokesman for catastrophe modeller AIR Worldwide told insuranceNEWS.com.au estimates for insured losses are not yet available.

“Many of the areas that flooded were outside the 100-year floodplain and were not considered at high risk,” the modeller’s Principal Scientist Hemant Chowdhary said.

“The historic flooding… is a combination of several factors including riverine flooding, backwater in tributaries due to high flood stages in main rivers, and significant local flash flooding caused by intense rainfall, flatter terrain and limited drainage capacity that was further exacerbated by backwater effects.”

US media reports say the floods, which began early this month, damaged at least 40,000 homes and claimed more than 10 lives in Louisiana and neighbouring Mississippi.

“The situation in Louisiana remains critical,” American Red Cross VP Disaster Services Operations and Logistics Brad Kieserman said.