Hamida Begum has fled her home on Bangladesh's Kutubdia island more times than she can remember while her neighbours have already given up the battle to prevent theirs being swallowed by the sea.

"I am scared our house will be washed away as well some day and during the monsoon season we can't live here at all," says the mother-of-four, standing outside the only one of a row of mud-brick shacks still intact.

"But this is our land and besides, we don't have money to go elsewhere."

Although around 100,000 people still reside on Kutubdia, few have any illusions they are living on borrowed time, with Coast -- a Bangladeshi NGO -- warning the whole island could disappear underwater within 50 years.

Tens of thousands have already left for good, mainly heading to the teeming capital Dhaka or a slum area of Cox's Bazaar, a resort town some 80 kilometers (50 miles) away.

In the build-up to the climate conference in Paris, there has been focus on low-lying island nations such as the Seychelles or those in the South Pacific which face obliteration if sea levels continue rising at current rates.

But their populations are dwarfed by the numbers living on the dozens of Bangladeshi islands and vulnerable coastal areas in what is one of the world's flattest -- and poorest -- nations.

"We have a long coastline, where about 39 million people live," Bangladesh's environment secretary Kamal Uddin Ahmed told AFP.

"If we have to shift those people to other areas it will be a big task for us because ours is a very densely populated country and we cannot really take all those people to other areas."

Bangladesh, along with the Philippines, Myanmar, and Haiti, is among the 10 nations most affected by the consequences of extreme weather events, according to a new climate survey released by advocacy group Germanwatch.

- Losing battle -

In 2009 the government set up a climate change trust fund, earmarking around six percent of the annual budget on adaptation measures.

On Kutubdia, authorities have erected a network of flood defences and stilted cyclone shelters where residents retreat during monsoons which turn what is a slice of paradise in the Bay of Bengal into a near warzone.

But they are fighting a losing battle, with Kutubdia's surface area having shrunk by around a quarter in the last three decades.

Many of the concrete blocks erected to stem the tide have collapsed or become buried under sand as water laps further inland.

"We have to move each time the waves come over and go and stay with a landlord who we have to pay rent to further inland. It's very painful for us," said Begum.

At least she still has a home, unlike Lutfun Nahar who has been living with her father since hers disintegrated during a storm three months ago.

"We often have to take shelter during the rainy season but this time it completely washed away my house," said the 43-year-old as she returned to view the remaining foundations.

"If there was a proper sea wall then perhaps I would rebuild but I am worried that if it happened again, I would have wasted all my money.

"I can't stay with my father forever as his home is not big and I have five children. But I don't know where we can go."

Around 40,000 islanders have relocated to Cox's Bazaar, eking out a living in corrugated shacks down rubbish-strewn alleyways.

Many still earn a living from fishing or boat-building. Others work in a dry fish processing centre where everything from baby sharks to pomfrets are hung out in their thousands. The stench is overwhelming.

- 'Nowhere to go' -

Nur Hussain made the move in 2012 and runs a cafe where fellow former islanders hang out.

"People now go fishing where my village used to be," said the 28-year-old.

"We used to have mosques, schools, madrassas, markets, many houses, -- all of that has been completely submerged... It is very painful for us."

Hussain shudders at the thought of moving again to make way for developers building a bewildering number of hotels serving one of the world's longest beaches.

"We are people of the sea. Everything we do is related to the sea... We have nowhere else to go."

That sense of dread at displacement is shared by small island nations whose populations may also have no option but to relocate.

Tuvalu Prime Minister Enele Sopoaga told AFP on a recent visit to India that his Pacific island nation might have to buy land in Australia and New Zealand to resettle its people.

Some of the atol nations have teamed up with larger countries such as Bangladesh most vulnerable to climate change, forming a so-called V20 as a counterpoint to the better known G20.

The last major climate summit in Copenhagen in 2009 left a bitter taste with the V20's leaders after rich counterparts baulked at making radical commitments to limit the rise in global temperatures to two degrees.

Moqbul Ahmed of Coast, who works with locals impacted by climate change, said Dhaka's government had a duty to look after those forced to flee but added that wealthy nations had a moral responsibility to cushion the blow.

"The richer countries should come forward to help our climate refugees," he said.