Floods have forced more than four million people from their homes across India, Nepal and Bangladesh and killed more than 100, as torrential rains in the initial days of monsoons wreaked havoc.

The poor Indian states of Assam and Bihar have been among the worst hit. Some 4.3 million people have been displaced from their homes in Assam in the past 10 days due to rising waters in the mostly rural northeastern region, according to a government release on Monday.

Television channels showed roads and railway lines in Bihar submerged, with people wading through chest-high, churning brown waters, carrying their belongings on their heads.

Floods in South Asia cause mass displacement and deaths annually, and the death toll and damage from the current monsoon season, which has just begun, is likely to increase in coming weeks. Floods in Nepal, India and Bangladesh during the 2017 monsoon killed at least 800 people and destroyed food crops and homes.

Local people gather concrete fragments and heavy bags wrapped in nets to build a dam as floodwaters flow from the north into the eastern Indian state of Bihar. (AFP/Getty Images)

An impoverished agrarian province with rickety infrastructure and poor health-care services, Bihar has a history of flooding in its northern areas bordering Nepal.

Flood waters in Assam rose overnight with the Brahmaputra River, which flows down from the Himalayas into Bangladesh, and its tributaries still in spate. 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.

"The flood situation has turned very critical with 31 of the 32 districts affected," Assam Chief Minister Sarbananda Sonowal told reporters. "We are working on a war footing to deal with the flood situation."

Assam, known for its tea industry, is hit by seasonal flooding each year, and the state and federal governments have spent millions of rupees on flood control.

Army and paramilitary personnel have been deployed across the state for rescue and relief operations and makeshift shelter camps have been set up, while the air force is on standby, Keshab Mahanta, Assam's water resources minister, told Reuters.

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

11 dead after building collapse in northern India

Eleven soldiers were among a dozen bodies recovered from the debris of a three-storey building that collapsed after monsoon rains hit a hilly area of northern India, officials said Monday.

Rescuers were looking for two to three people still unaccounted after Sunday's collapse, said Gaurav Srivastav, an official from the National Disaster Response Force, or NDRF.

Indian rescuers rushed to the scene after a multi-storey building collapsed following heavy rains near Kumarhatti, India. (AFP/Getty Images)

Fire official Raja Ram Bhagate said the dead included 11 army soldiers who were having a party in a ground floor restaurant in the building when it collapsed. One civilian also was killed.

Several soldiers were among the 31 people rescued after the collapse occurred in Solan, a town in Himachal Pradesh state. The area is 310 kilometres north of New Delhi.

More than 70 NDRF rescuers and 40 fire officers have been clearing the rubble, using earth movers, drillers and gas cutters.

Building collapses are common in India during the June-September monsoon season, when heavy rains weaken the foundations of structures that are poorly constructed.

Fatal landslides in Nepal

In neighbouring Nepal, 64 people were killed and 31 were missing, with around a third of all districts hit by heavy rains, authorities said. Many of the deaths were caused by landslides that swept away houses.

In southeast Nepal, water levels on the Kosi River, which flows into Bihar, had receded, an district official said.

In 2008, the Kosi broke its banks and changed course, inundating huge tracts of land and killing 500 people.

"Our analysis is that the danger is over now that the water level has come down," Chiranjibi Giri, assistant district administrator of Sunsari district, told Reuters.

A woman struggles to walk along a flooded street in Lalitpur, Nepal, amid monsoon season. (Navesh Chitrakar/Reuters)

In Bangladesh, floods forced an estimated 190,000 people out of their homes, government officials said.

In Cox's Bazar district, shelter to some 700,000 Rohingya refugees who fled violence in neighbouring Myanmar, more than 100,000 people have been displaced.

Since early July, flooding and landslides have damaged thousands of shelters at the refugee camps, killing two people, including a child, Human Rights Watch said in a release last week.