More than 100 people have been killed and millions more affected by devastating floods and landslides across parts of south Asia.

Heavy monsoon rains over the past week have left many dead in Nepal and Bangladesh, and submerged vast areas of north-east India.

In the north-eastern Indian state of Assam, among the worst-hit areas, agencies were working on a “war footing” to deal with the situation, the chief minister, Sarbananda Sonowal, said.

About 4.3 million people in Assam have been affected by flooding, while 83,000 have been forced to seek shelter in relief camps on higher ground.

Television footage in India showed people wading waist-high through flooded roads and elderly people being carried to safety by rescue teams.

In Bihar, east India, a further 2.56 million people were hit by flash floods, with many homes quickly submerged in brown water.

“Floods have turned our homes into a hell,” said Rupesh Jha, who lives in Araria district in Bihar. “We are somehow eating and using makeshift boats to go outside to defecate in the flood water as we have no alternative,” he said.

“What has further added to our problem is that there has been no electricity in our homes for the past one week since waters flooded our homes,” he said, adding that food supplies from government were lacking.

Floodwater entered his home on 12 July, he said. “Very soon we were left in the waist deep water.” Many of his neighbours fled their homes with their cattle and few belongings.

A family bring their belongings to a makeshift camp in Morigaon district of Assam. Photograph: STR/EPA

Every year, floods cause deaths and force huge numbers of people to flee their homes across south Asia. It is feared the death toll and damage from the monsoon season, which has just begun, is likely to increase in coming weeks.

Floods in Nepal, India and Bangladesh during the 2017 monsoon killed more than 1,000 people and destroyed food crops and homes.

“We have taken shelter in schools, temples and other higher places as our homes are flooded. We are very worried about our belongings left in our homes since there are none to protect them,” said Rakesh Kumar Singh, from Sitamarhi district in Bihar.

Most of his neighbours’ homes have been destroyed. “We are somehow surviving on government midday meals at a local school,” Singh said. Midday meals are usually offered free to children enrolled in government schools, but are being rolled out to communities affected by floods.

Locals gather building materials to build a dam as floodwaters flow from the north into Bihar near Muzaffarpur. Photograph: STR/AFP/Getty Images

Bihar experiences floods annually, but this year has been struck by flash flooding, said Pratyaya Amrit, principal secretary of the state’s disaster management department.

“Floods have always been a challenge for us and we have faced it bravely. This time too we will overcome the problem,” he said.

A staff member working on the response in Assam said the numbers of people affected were higher than last year. Emergency provisions – such as rice, dal, baby food, water purification tablets and sanitary products – were being handed out at distribution points and relief camps. Water pouches were also being distributed in remote areas.

Villages in Assam were deluged when the Brahmaputra River, which flows through India, Bangladesh and China, burst its banks. Minazur Rahman, a primary school teacher from Fakirganj in Assam, said: “If the water levels do not go down then we will be in very big trouble.” Many of his neighbours, whose homes are submerged, are staying at a crowded relief camp at a school, three hours away.

“The river has already broken my house once in the past,” he said, adding that he feared the same would happen again unless the government took action.

Most of the Kaziranga national park, home to the rare one-horned rhino, was underwater, authorities in Assam said, adding that four people drowned on Monday.

Children paddle a raft through flood waters at Kalgachia in Barpeta district in India’s north-eastern state of Assam. Photograph: David Talukdar/AFP/Getty Images

The Indian weather office has forecast widespread rains across Assam and Bihar over the next two days.

In neighbouring Nepal, 78 people have been killed and 31 are missing, with about a third of all districts hit by heavy rains, authorities say.

Across Bangladesh, the devastating rains have affected tens of thousands of people. Last week, dozens of people, mostly farmers in rural areas, were killed by lightening.

The severe weather has also hit Cox’s Bazar, in southern Bangladesh, where about 700,000 Rohingya refugees who fled violence in neighbouring Myanmar, are living in refugee camps.