Bush has held up the Millennium Challenge Account as a landmark programme to provide badly needed assistance to nations that undertake political and economic reforms.
In his original proposal in March 2002, he proposed $5 billion for the programme by 2006.
Yet three years later, the programme has only committed $110 million to Madagascar and $215 million to Honduras. On Monday, the programme's board approved $175 million for Nicaragua and $110 million for Cape Verde.
But of that money, only $117,500 has actually been paid out.
"I assured the leaders we will work harder and faster to certify countries from MCA (Millennium Challenge Account), so that MCA countries and the people in the MCA countries can see the benefit of this really important piece of legislation and funding," Bush said.
Bush's comments came after he met five African leaders privately - presidents Festus Mogae of Botswana, John Kufuor of Ghana, Armando Guebuza of Mozambique, Hifikepunye Pohamba of Namibia and Tandja Mamadou of Niger.
They also discussed progress in the fight against HIV/Aids, which many consider their biggest development challenge. Sub-Saharan Africa is the worst-hit by the disease, which in countries such as Botswana affects one in three of the adult population.
The Botswana leader said the presidents of Niger and Ghana complained about the Millennium Challenge Account in their talks, telling Bush there were too many bureaucratic hurdles.
"On the part of the United States, it's often the bureaucracy and the fine print sometimes putting conditions which makes it impossible or very difficult for us to access what is otherwise available to us," Mogae said.
Bush, Blair differences
Bush went into the meeting having turned down a proposal by British Prime Minister Tony Blair to give Africa as much as $50 billion a year by making long-term aid commitments that would allow poor countries to raise money on global capital markets.
Blair, whose country holds the rotating presidency this year of the G8 grouping of wealthy countries, is still pushing for other nations to join in his plan, after declaring 2005 a make-or-break year for plans to lift Africa out of poverty.
Poverty and Aids threaten the
lives of millions in Africa
Bush and Blair did work on a debt relief plan for Africa that finance ministers agreed on in London over the weekend, before a Group of Eight summit next month in Gleneagles, Scotland.
Under the deal, about $40 billion in debt owed by 18 of the world's poorest nations, including 14 in Africa, will be cancelled.
"We believe by removing a crippling debt burden, we'll help millions of Africans improve their lives and grow their economies," Bush said.