The Group of Eight (G8) - Britain, Japan, Canada, the United States, France, Italy, Germany and Russia - announced a deal on Saturday to write off all the multilateral debts of some of the world's poorest countries.
Relief agencies said it was the type of move they had been campaigning for, but the G8 could go further and expand the list of countries to benefit as well as the basket of relief offered.
They also said aid often failed to reach the poorest.
"Very substantial bottlenecks exist in moving funding from commitment to actual help. Sometimes such gains do not reach those who need them the most," said Greg Ramm, head of the UK charity Save the Children in southern Africa.
His group wanted to see the gains from debt relief go to improving schools, healthcare and provision of food - especially for children, Ramm said.
He told British television on Sunday: "It is the biggest debt settlement that can be reached and I believe that that is the starting point.
"But it's only going to succeed if it is matched by aid, by trade justice, by transparency, by tackling corruption, by dealing with the governance issues."
The $40 billion offered by the G8 in debt relief would be enough to wipe out debts owed to international organisations by 18 countries which are at the so-called completion point of an IMF and World Bank initiative.
"It is the biggest debt settlement that can be reached and I believe that that is the starting point. But it's only going to succeed if it's matched by aid, by trade justice, by transparency, by tackling corruption, by dealing with the governance issues"
Save the Children
The completion point is the date when the scheme's debt write-off becomes effective. The 18 countries are: Benin, Bolivia, Burkina Faso, Ethiopia, Ghana, Guyana, Honduras, Madagascar, Mali, Mauritania, Mozambique, Nicaragua, Niger, Rwanda, Senegal, Tanzania, Uganda and Zambia.
But analysts said the debt stock for some of the countries owed to other lenders rather than to the international organisations was still too large.
"We need the entire debt to be written off to make an impact on economic growth and development and to accelerate spending in social sectors like health and education in a country like Zambia," said Isaac Ngoma, executive secretary of the Lusaka think-tank Economics Association of Zambia.
"For many of the countries involved it is a positive step, but it is also a small step," Ngoma said.
Other aid workers said the G8 had shown that it could take drastic steps to help Africa and next month's G8 summit in Gleneagles, Scotland could still be significant by showing what powerful countries could do.
"There are 60-odd countries that could use debt relief as a first step and eventual debt cancellation as the ultimate action required to help," one aid worker said in Johannesburg.
"For many of the countries involved it is a positive step, but it is also a small step"
Economics Association of Zambia
South Africa's Archbishop Desmond Tutu urged officials to monitor the deal closely to ensure funds were not diverted by corrupt leaders.
In Maputo, Mozambique Prime Minister Luisa Diogo said more effort was needed to increase aid and reduce trade imbalances, but the G8's debt action had left her country with only "a very small amount of debt" to other lenders.