Rich countries are meeting in Berlin on Thursday to pledge cash to smooth the way for a global climate change deal.
The Green Climate Fund is designed to help poor nations adapt to climate extremes like droughts and floods and to buy low-carbon energy sources.
Rich nations previously vowed that by 2020, developing countries would get $100bn (£64bn) a year from such a fund.
The Berlin conference aims to create a focus that will embarrass governments to come forward with contributions.
The fund is supposed to hold at least $10bn by the end of 2014.
President Obama has pledged $3bn, Japan has offered $1.5bn, and France and Germany have also offered significant sums.
It is thought that the UK will pledge around $1bn from existing aid budgets.
'Our moral duty'"The poorest and most vulnerable on the planet are already suffering the effects of climate change and it's our moral duty to act," UK Secretary of State for Climate Change Ed Davey told the BBC.
He said the aid could save lives by protecting low-lying islands and coastal settlements from the impact of rising sea levels and helping farmers hit by the weather effects of climate change.
"I urge other countries to be equally ambitious," he added.
UK Prime Minister David Cameron has been under pressure from critics who say the UK should spend the money helping combat the effects of extreme weather at home.
Benny Peiser from the fossil fuel lobby group GWPF said international climate finance for low carbon development was "a detrimental use of aid money".
"The international community should be encouraging the development of the cheapest forms of electricity generation that offer populations in the developing world the best chances of escaping poverty," he said.
The fund was agreed because developed nations have caused the majority of global warming so far - and their CO2 emissions stay in the atmosphere for 100 years. Poor countries asked for help to adapt to climate change they have not caused.
As greenhouse gas emissions are a global problem, rich nations acknowledged a degree of self-interest in helping developing countries to invest in clean technology too.
'A drop in the ocean'Asad Rehman from Friends of the Earth said: "Helping poorer nations to develop their economies cleanly and without wrecking our atmosphere is in all of our interests. As one of the nations most responsible for historic carbon dioxide emissions, the UK should be taking a lead in tackling climate change."
But he said the UK's pledge was "a drop in the ocean compared to what is desperately needed".
"Britain's contribution must also be new and additional money - not raided from existing aid pledges. Some of it should come from the hundreds of millions of pounds they hand over every year to dirty energy corporations," he added.
The middle income countries of Korea, Indonesia and Mexico have made voluntary contributions to the pot - Korea has pledged $100m. The climate-sceptic Australian government says it will not commit money.
Pa Ousman Jarju, Gambia's environment minister, said he wanted to hear significant pledges come out of Berlin.
"We expect a minimum of $15bn to come out of the pledging conference. We are glad that, with the recent US and Japan pledges, it has reached around $7.5bn," he said.
There has already been acrimony over the tardy contributions from some rich nations - and Mr Jarju warned mistrust would increase if rich countries failed to make proper contributions.
Rich nations and poor alike need each other if the world is to reach a global climate agreement scheduled for Paris next December.