President George W Bush has proposed doubling US aid to Africa over the next five years.
He said this would happen if African leaders made a commitment to honest government and the rule of law.
Outlining his priorities for the G8 summit next week, Mr Bush said the West now had an extraordinary opportunity to help end extreme poverty in Africa.
White House adviser Stephen Hadley said aid would rise from $4.3bn (£2.4bn) in 2004 to $8.6bn by 2010.
Congress has not yet approved Mr Bush's proposed increase in aid.
But on the other main issue facing the summit - climate change - Mr Bush gave no indication of a compromise.
The president criticised those who opposed energy development and wanted to place restrictions upon it.
"About two billion people have no access to any form of modern energy," he said.
"Blocking that access would condemn them to permanent poverty."
Agents of reform
Mr Bush said the US would double assistance to the region by 2010, but stressed trade and good government were as important as financial aid.
WHAT IS THE G8?
Group of eight major industrialised states, inc Russia
Canada, France, Germany, Italy, Japan, Russia, UK, US
Originally set up to discuss trade and economic issues
Now leaders discuss global issues of the day
2005 Summit agenda
He said the "primary focus" would be on "reforming countries".
African leaders, he said, must be the "agents of reform" rather than "passive recipients of money".
The announcement is in addition to $674m (£350m) in aid for Africa promised by Mr Bush in a summit with UK Prime Minister Tony Blair earlier this month, at which the writing off of debts owed by some of the poorest countries was agreed.
US government development aid is lower than most Western countries when measured in terms of gross national product, but its non-governmental donations are much higher.
In his speech, Mr Bush said there had to be a new approach to dealing with the debt burden of the poorest countries.
He said there must be an emphasis on encouraging trade, which he called the "engine for development".
The dismantling of trade barriers and wider opening up of Western markets to African products has been a key demand of anti-poverty campaigners.
South African presidential spokesman Bheki Khumalo told the BBC he was "very pleased" with Mr Bush's announcement on aid.
It was, he said "a step in the direction we want. It is now incumbent upon Africans themselves to improve on all aspects of governance".
Mr Blair's office said Mr Bush's pledge "creates real momentum for a successful outcome at Gleneagles," the Scottish venue for the G8 summit of the world's richest nations.
But the anti-poverty charity ActionAid said Mr Bush did not go far enough.
"This is a very modest step forward that is being spun as a colossal leap," said spokesman Patrick Watt.
It is also thought critics will look carefully at how the figures in the speech translate, as money pledged is not necessarily the same as the sums the US Congress will approve.