Torrential monsoon rains, overflowing embankments and burst dams have unleashed massive flooding in South Asia, killing more than 1,500 people and forcing several million from their homes in India, Nepal, Bangladesh and Pakistan.

The north and east of India have been hit particularly hard with the state of Bihar seeing its worst floods in 50 years. The state was inundated when the Koshi river in neighbouring Nepal breached its banks and changed course.

The river, known as the Kosi once it gets to India, has submerged hundreds of villages in both Nepal and India. Authorities in India have ordered a probe to determine whether negligence contributed to the disaster.

The flooding has forced some 3 million people from their homes in Bihar, one of India's poorest states. Officials say around 250 people have been killed, more than 300,000 houses destroyed and at least 840,000 acres (340,000 ha) of crops damaged.

Tens of thousands are living in government camps, but hundreds of thousands still remain stranded in their villages and sending relief to them has been hampered as floodwaters have washed away roads in most places.

In neighbouring Nepal, the floods have displaced more than 100,000 people, the U.N. Children's Fund, UNICEF, estimates. And scores have been killed since the monsoon rains began in June.

Aid agencies say the flooding of Nepal's fertile Eastern Terai region spells disaster for the whole country. The population are mostly farmers who provide rice and vegetables to other parts of the country. Not only has the deluge destroyed their harvests, it has also damaged the highway that is vital for transporting produce to other regions. The floods effectively cut the country in half. Click here for an AlertNet article on the flooding in Nepal.

Nepal has urged foreign donors to provide food and medicines to thousands of people left homeless.

Elsewhere in India, more than half a million people fled their homes in Orissa, when large parts of the coastal state were flooded after authorities were forced to open the sluice gates of a dam on the Mahanadi river due to heavy rains. The rising waters of the Mahanadi and its tributaries have broken through mud embankments and swamped hundreds of villages.

Doctors and health workers are struggling to treat thousands of villagers suffering from water-borne diseases in Orissa and Bihar. Many are children who have been forced to drink dirty flood water to survive.

In the northern Indian state of Uttar Pradesh, flooding has killed more than 1000 people and affected 2.3 million - as well as crops and livestock.

Assam in the northeast has been hit too, with more than a million people affected by floods there after the Brahmaputra river burst its banks. About 120,000 hectares of farmland was submerged, and at least 15 people killed. Most of the Kaziranga National Park, home to more than half of the world's population of one-horned rhinoceros, ended up under water.

In cyclone-prone central and northern Bangladesh, about 200,000 people have been displaced and more than 100,000 hectares (250,000 acres) of farmland submerged.

In India, experts and aid agencies have blamed the government for failing to warn people ahead of the flooding in Bihar and for mishandling relief work.

Emergency fax messages sent by engineers warning of an impending disaster were ignored in Bihar's capital Patna, weeks before the disaster struck, according to a disaster management official. The faxes piled up on one bureaucrat's desk because he was on leave and no deputy had been appointed, officials say.

Some people have also accused India of failing to maintain the embankments under a treaty signed with Nepal 50 years ago.

ANNUAL DELUGE

Monsoon seasons in South Asia bring severe storms and flooding every year to wide swathes of Bangladesh, India, Nepal and Pakistan. But climate change is now making things worse, as melting glaciers in the Himalayas increase the risk of flooding but cut the supply of fresh water.

The monsoon is essential for the region's agriculture, but poor infrastructure and poverty have left communities increasingly ill-equipped to cope with the impact of heavy rains. Flooding and landslides claim lives, destroy property and crops and increase the prevalence of diseases such as malaria and dengue fever.

The seasons can be loosely categorised into the northeastern winter monsoon which runs from December to early March, and the southwestern summer monsoon, which normally lasts from early June to early September and supplies more than 80 percent of annual rainfall.

The winter monsoon - which blows in from the northeast carrying moisture from the Bay of Bengal - is also normally responsible for cyclones in India.

Environmental scientists say South Asia, and India in particular, is set to be one of the worst-affected regions by climate change as ever rising temperatures melt the Himalayan glaciers. More than 500 million people - almost half of India's total population - living in the Indus, Ganges and Brahmaputra river basins rely on the glaciers for their water supply.

Paradoxically, faster melting glaciers increase the risk of rivers bursting their banks because of the larger volumes of water, but the fact that they are also receding means that water supplies aren't reaching as far as they used to.

Not only that, experts say rising temperatures are affecting moderate summer monsoon rainfall which is normally easily absorbed by the earth and recharges the ground water. Instead, rain now falls in heavier bouts on drier ground, which is more prone to flooding and landslides.

According to the Indian Institute of Tropical Meteorology, so-called extreme rain events could reach 100 per year in India, more than double the 45 that were common in the 1950s. Researchers also say rising temperatures will mean mosquito-borne diseases will spread to higher altitudes.

The worst flooding in recent years hit areas of India's Maharashtra state around Mumbai in July 2005. According to the International Federation of Red Cross and Red Crescent Societies, more than 22 million people were affected, including 3,400 killed, 1,100 people injured and another 450,000 displaced.