Two problems stemming from the 2008 financial crisis — heavy government borrowing and high unemployment — still pose challenges to the global economy and require bold action, the head of the International Monetary Fund says.

Christine Lagarde, IMF managing director, says cheaper oil and strong U.S. growth aren't enough to counter those threats.

"We believe that global growth is still too low, too brittle and too lopsided," she said in remarks before the Council on Foreign Relations.

Europe and Japan potentially face years of slow growth and ultra-low inflation, she said, while the United States "is the only major economy that is likely to buck that trend this year."

Lagarde says the threat of deflation in Europe "bolsters the case" for the European Central Bank to provide more stimulus.