"Entire villages are days away from a health crisis if people are not reached in the coming days," UNICEF's health chief for India, Marzio Babille, said in a statement.

World Health Organisation (WHO) spokeswoman Fadela Chaib said acute respiratory and skin infections, food-borne ailments and snake bites were also threatening the 30 million people affected by flash flooding in the region.

Many people in affected areas are relying on dirty surface water for their basic needs, with water sources contaminated or still submerged in the wake of the floods.

"The main problem is access to clean water and sanitation," Chaib told a news briefing, noting that UN agencies, other aid groups and governments were working to ensure water, rehydration salts and other medical supplies reached those in need.

Many remote communities and villages are only accessible by boat or through air drops, and security concerns in some areas has further hindered efforts to deliver humanitarian aid, UNICEF spokeswoman Veronique Taveau said.