Finance is a hot-button topic at the annual round of UN climate talks underway in Marrakesh, Morocco -- the first since the world's nations adopted a hard-fought agreement last year to rein in global warming from fossil fuels.

Rich nations pledged back in 2009 to mobilise USD 100 billion (93 billion euros) per year from 2020 for climate aid to the developing world.

But poor nations are insisting on a clear roadmap for reaching the goal, and on a bigger focus on money for climate adaptation, which entails building resilience against global warming's impacts.

So far, though, adaptation has received less than a fifth of climate finance made available.

"The cost for developing countries of adaptation alone could range from USD 140 billion to USD 300 billion by 2030, and from USD 280 billion to USD 550 billion by 2050 -- up to five times higher than previously estimated," UN Environment -- also known as UNEP -- said in a statement.

"The world has promised to help developing countries adapt to the disruption and hardship caused by changes such as altered rainfall patterns and rising sea levels," the agency's head, Erik Solheim, said in a statement issued in Marrakesh.

"We must keep that promise, narrow the adaptation finance gap and prevent it from widening further by redoubling our efforts to stabilise the world's climate."

Money for adaptation could go towards building dykes and homes elevated higher than projected sea-level rise, putting in place weather warning systems, improving access to drinking water, and making available seeds for climate resistant crops.

According to the Organisation for Economic Cooperation and Development (OECD), pledges made by rich countries in 2015 alone would increase public climate finance from USD 41 billion in 2013-14 to USD 67 billion in 2020.