She said she believed Chinese officials understood the imperative to reduce the debts of some of these countries, but declined to forecast what China would ultimately do, and when.
Yellen, long critical of the pace of China's efforts on debt treatments for Zambia and other countries, on Friday called again for China and other countries to provide "timely," "comprehensive" and "meaningful debt relief to help countries regain their footing."
Yellen said U.S. officials expressed specific concern about Zambia, whose debt restructuring effort under the Group of 20 Common Framework has taken much longer to resolve than expected.
Failure to act would result in negative spillovers, including conflict, fragility, war, terrorism and migration, she said, sucking up resources that would hamper a country's ability to grow and move forward, Yellen said.
Partial debt reductions would allow a country to invest and grow and pay back some of the reduced debt, she said.