Some villagers began trying to return to their flooded homes in eastern India on Saturday as waters slowly receded, but officials warned the move was risky with a month of heavy rains still expected.

Almost 900,000 people have been evacuated to higher ground since flood defence walls broke upstream in Nepal almost three weeks ago, shifting the flow of the Kosi river away from its normal course and east onto farmland.

Large swathes of the impoverished state of Bihar were flooded. About 100,000 people remain marooned in village islands by the river, with most refusing to leave, while some evacuees are trying to head home.

"We have reached a stage where people in the thousands are still left, but they are now refusing to come out," state disaster management official Pratyaya Amrit told AFP.

"People think the flood is over. In a lot of the camps people have started moving back. In the last two to three days, at least 10,000 to 15,000 have gone back."

But India's monsoon season, when much of the country receives more than 90 percent of its rain, does not end till the close of September, and officials say the river could rise again.

About 1,100 square kilometres (440 square miles) of villages and farms remain underwater, even though levels have decreased by a foot (30 centimetres) or more in parts.

In areas surrounding Birpur town, on the border with Nepal, an AFP photographer accompanying an Air Force relief flight said more than 80 percent of the area was under water.

On Saturday, some 2,000 people could still be seen Saturday sheltering on rooftops and dry strips of land, waving to the helicopter for emergency food supplies.

The helicopter was dropping red and orange water-resistant packages containing beaten rice, lentil flour, palm sugar and water purifying tablets, a relief official said, adding that the relief sorties would be reduced Sunday.

"Right now we have nine helicopters," Deepak Kumar Sahu, an official coordinating flood relief efforts told AFP.

"Tomorrow we'll withdraw two. The focus is on the evacuation and the rehabilitation camps."

Army officials running boat rescue operations this week said the receding water was making it harder to reach distant villages, where people have been without food or water for weeks.

"Beyond 15 to 20 kilometres (nine to 12 miles) it is hard for us rescue," said one army official, asking not to be named.

"We're trying to get as far as possible."

But with frequent stops to pull boats across shallow sections of water, they can only get as far as nearby villages where people want to stay put, asking instead for supplies to be sent to them.

On a road in Bihar's Purnia district, 350 kilometres from state capital Patna, rescued villagers unloaded from boats trudged towards the town and camps all week.

But as reports circulated that water levels were falling, just as many villagers were walking the other way, to check how much water was on either side of the road and decide what to do next.

With camps running out of room for the tens of thousands of people who have left their homes, many of the freshly evacuated villagers are living on the road.

But local administrative officials say new camps were being set up and more medical teams were being deployed.

"We're increasing capacity by putting up tented accommodation," said Purnia's top administrative official C. Sridhar.

"Some people who are coming with cattle are setting up on the side of the road. We are persuading them to move towards camps."

Disaster official Amrit said rescue boats would ferry food to villagers as well as try to convince them to evacuate.

"We have another 25 days (of monsoon) to go. What if the levels go up again?"