NEW DELHI, India (CNN) -- Monsoon rains in South Asia have driven millions from their homes and caused what the United Nations says is the worst flooding in living memory.
A villager transports his mother to safety on a raft through floodwaters on Thursday near the Kaziranga National park in India.
More than 1,000 people have been killed or injured by rising waters, but aid agencies say the figure is expected to rise sharply.
U.N. children's body UNICEF said it had lost track of how many people had been affected by the floods across India, Pakistan, Bangladesh and Nepal.
So far about 20 million people are known to have fled their homes or trapped in villages at risk from landslides, snakebites and disease.
"Hundreds of thousands have lost their homes, their possessions, livestock and fields and will have to begin their lives from scratch when flood waters recede," UNICEF said.
The devastation comes on the heels of severe flooding in southern Pakistan, caused when Cyclone Yemyin struck the country's provinces of Balochistan and Sindh in late June.
India appears to have been hardest hit by the latest inundations with floodwaters striking the densely-populated and poor states of Bihar and Uttar Pradesh.
"According to government estimates, the cumulative number of human casualties stands at 1,103 in 138 affected districts and more than 112,000 houses have been damaged or destroyed though these figures are set to rise as the situation unfolds," UNICEF said. It was not clear whether the casualty figure includes injured as well as dead.
Authorities in Bihar say at least 30 people have died amid the rains and flooding.
"Nearly 180 relief camps have been set up in different parts of the state. Army helicopters will be pressed into service to distribute food packets from Saturday in some parts of the state," says Manoj Srivastava, the state's Disaster Committee chairman.
Bihar state governor, R. S. Gavai, has appealed for political parties to work together to aid flood victims.
Up to 500,000 people are believed to have been affected by floods in India's northeastern Assam, the state's Water Resources Minister, Bharat Chandra Narah, told the CNN.
"Nineteen people have been confirmed dead. Relief materials are being distributed in certain areas in the state. Nearly 500 relief camps have been set up," Narah said.
All but two of the state's 27 districts have been affected and vast areas of land, including many farms, are under water, Narah said. Assam's Brahmaputra river was flowing above the danger mark in some areas.
Rains have also affected major cities of Mumbai and the Indian capital of New Delhi.
In Nepal, 84 people have been killed by flooding and landslides, and more than 9,700 families have been displaced with 270,000 people in 32 districts affected in the last two weeks, UNICEF said
Dharmaraju Kakani, a senior official for UK charity Oxfam, said at least 28 out of 75 districts in the country have been affected.
"At least 96 people have died so far in flooding in Nepal and nearly two hundred thousand others have been affected," Kakani told the CNN. Landslides are also affecting relief operations in some areas.
In low-lying Bangladesh, the government says relief efforts are going on to help millions affected by floods.
"The government along with the NGOs (non-governmental organizations) are undertaking relief operations and trying to help people," a senior Foreign Ministry official, Nazmul Kawnain told the CNN.
UNICEF said there are fears that the eastern side of the capital of Dhaka "could be inundated in the next few days as waters make their way towards the lowlands."
Access for rescuers will be difficult because rising waters continue to inundate flooded terrain, UNICEF said. E-mail to a friend