26 NDRF teams are carrying out rescue operations in Bihar. (AFP)

Flash floods caused by heavy rains in catchment areas of Nepal have ravaged a dozen districts of Bihar so far, claiming 78 lives and affecting 55 lakh people, the state's disaster management department said on Thursday.

Sitamarhi reported the highest number of 18 casualties, followed by Madhubani (14), Araria (12), Sheohar and Darbhanga (nine each), Purnea (seven), Kishanganj (four), Supaul (three) and East Champaran (two), it said in a release.

The number of deaths in Bihar rose by 11 since Wednesday.

More than one lakh people are staying at the relief camps in Sitamarhi, while 3,721 have taken shelter at Madhubani, the department said.

The state government has installed 1,119 relief camps, spread across all the 12 affected districts, to ensure proper food and safe drinking water to those impacted by the deluge.

A total of 26 teams of the NDRF and the SDRF are patrolling the flood-hit areas, in addition to 796 personnel devoted to relief and rehabilitation work. They have been equipped with 125 boats for the purpose, the release said.

A department official said there has been a let up in rain in Nepal, which has caused water levels to recede, though as many as six rivers flowing through the state are still above danger mark at many places.

Scenes of distress have been emerging from the inundated areas, with news channels flashing images of people carrying children in baskets on their heads in neck-deep waters.

A video shot on a mobile phone, of a building collapsing like a pack of cards upon being hit by a gushing stream of flood water in Sitamarhi, has also gone viral.

At many villages, locals have built bridge-like structures with the help of bamboos to cross canals.