The huge sum - largely spirited away by leaders and their associates - was one of the main reasons why Africa's poverty was so severe.

The Western world must demonstrate practical commitment to assist us by repatriating monies that have been stolen from our treasuries and stashed away in their financial institutions

Olusegun Obasanjo, Nigeria's president

Now, Mr Obasanjo told a meeting of civil society organisations in Ethiopia's Addis Ababa, it was time to write rules to help bring some of the money home.

"We are working to get an international convention by which money stolen by corrupt African leaders and stashed abroad is repatriated," Mr Obasanjo said.

Mr Obasanjo said that while the leaders were the main culprits, Western countries which had harboured the stolen loot should bear some responsibility.

"It is not enough to accuse developing countries of corruption," he said.

"The Western world must demonstrate practical commitment to assist us by repatriating monies that have been stolen from our treasuries and stashed away in their financial institutions."

Dirty money

Nigeria has suffered more than most African countries from the tendency of its leaders to treat its economy as a personal cash cow.

Some of Abacha's plundered billions have been returned

While most post-independence leaders have been accused of greater or lesser levels of corruption, its last dictator but one, Sani Abacha, proved the most publicly egregious.

Abacha, who died in office in 1998, and his family squirreled billions of dollars away in foreign bank accounts.

About $1bn has now been returned, the bulk of it from Switzerland.

But much more remains either unaccounted for or - in the case of cash frozen in UK bank accounts - unreturned pending legal action.

The deal to return the money, however, left about $200m in the hands of the Abacha family - a "very difficult" compromise to agree, Mr Obasanjo said.

"If there is an international convention in place, it would have been easier to recover such monies."

Across the board

Nigeria is far from alone in having suffered from its leaders depredations.

For instance in Zaire, now the Democratic Republic of Congo, billions were looted through exploitation of mineral revenues during the 30-year rule of Mobutu Sese Seko.

In Angola, pressure groups estimate that as much as $1bn a year in oil-related revenue disappears every year.

A new initiative, spearheaded by international financier George Soros and the NGO Global Witness, is trying to persuade oil and mineral companies to publish full accounts of what they pay in tax and other levies so as to shine a spotlight on this kind of theft.