IMF Managing Director Christine Lagarde said stimulative policies are still needed in key regions, especially Europe and Japan, which have struggled with prolonged weakness.
She spoke at an annual economics conference in Jackson Hole, Wyo., sponsored by the Kansas City Federal Reserve Bank.
Lagarde said central banks must carefully develop strategies for scaling back their efforts to keep borrowing rates low. Any pullback should be determined by the strength of individual economies, she said.
Her comments come as the Fed is signaling that it could slow its bond purchases later this year if the U.S. economy continues to improve. The Fed's bond buying has helped keep U.S. interest rates near record lows.
"Unconventional monetary policy is still needed in all places it is being used, albeit longer for some than for others," Lagarde said in her speech to the conference.
The anticipation of a slowdown in Fed bond buying has unsettled U.S. stock and bond markets and sent interest rates up. Rising U.S. rates have, in turn, triggered turmoil in some emerging economies, such as Turkey, India and Indonesia. Officials in those countries have tried to halt declines in the value of their currencies as investors have shifted money into higher-yielding investments elsewhere.
Lagarde noted the market declines that have followed Fed Chairman Ben Bernanke's signal in June that the Fed could begin slowing its bond purchases later this year if the U.S. economy strengthens further.
She said finance officials should prepare contingency plans in case market turbulence worsens.
Some investors think the Fed could announce at its next meeting in September that it's reducing its bond purchases. But comments from Fed officials at Jackson Hole suggested some disagreement within the central bank over the proper timing for a slowdown to begin.
Dennis Lockhart, president of the Federal Reserve Bank of Atlanta, suggested in an interview with CNBC that he might be ready to endorse a bond-buying slowdown in September. But James Bullard, president of the St. Louis Fed, said he thought the economy remains too uncertain for a pullback next month.
"I don't think we have to be in any hurry," Bullard said in a separate interview with CNBC. "I think we want to take our time and assess what is going on."
In a separate interview, Bullard said the decision on whether to start tapering could well come into focus just a few days before the Sept. 17-18 meeting if Fed Chairman Ben Bernanke decides at that time to provide the other policymakers with a recommendation.
"That's probably the kind of thing that it would come down to just a few days before the meeting, he'd kind of lay out some options," Bullard said in an interview with Fox Business News.
Bullard is a voting member of the Fed's interest rate panel this year. Lockhart takes part in discussions but doesn't have a vote this year. Their remarks mirrored the divided opinion that was evident in the minutes of the Fed's July meeting released this week.
In her speech, Lagarde said the support being provided by major central banks is buying time for nations to implement key economic reforms.
"Push ahead with deeper reforms to lay the foundation for durable and lasting growth," Lagarde said. "Do not waste the space provided by unconventional monetary policies."
For example, she said troubled nations in Europe must repair their financial systems before credit can start flowing normally again.
Lagarde said some emerging market countries have taken steps to prepare for the shocks that could occur as the United States and other major economies withdraw their extraordinary support and borrowing rates rise to historically normal levels. But she said more work would be needed.
She said the IMF will provide support where possible, including emergency loans to countries that need them.
After her speech, Lagarde said she hoped leaders of the Group of 20 major economies will seek to complete financial reforms that have been drafted to try to avoid another crisis like the one that erupted in 2008. The G-20 leaders will meet next month in St. Petersburg, Russia.
"As the level of the crisis has abated, the level of urgency has waned a bit," Lagarde said in response to a question.
Associated Press writer Marjorie Olster contributed to this report from Washington.