Jeffrey Sachs, special adviser to U.N. Secretary General Kofi Annan on anti-poverty targets, said Africa's heavy debt burden was untenable and urged the continent not to pay its debts if rich countries refused to cancel them. The U.S. economist, director of the Earth Institute at Columbia University, spoke at a conference in Addis Ababa, Ethiopia, on hunger on the eve of a summit of the heads of state of the African Union, which estimates sub-Saharan Africa's foreign debt at $201 billion.