WASHINGTON (CNNMoney.com) -- Treasury Secretary Timothy Geithner --  in remarks that appeared aimed toward China -- took a sharper tone on  the issue of currency Wednesday, saying global economic growth could be  at risk unless developing nations are more flexible about undervalued  currencies.
Without mentioning China by name, Geithner said tight  controls on currency risk "either causing inflation and asset bubbles in  emerging economies, or else depressing consumption growth and  intensifying short-term distortions in favor of exports," in a speech at  the Brookings Institution in Washington.
Geithner said that a main U.S. goal going into International Monetary  Fund meetings in coming days is to "convince" those nations controlling  their currencies to be more flexible.
"It is not good for the  world for the burden of solving this problem to rest on the shoulders of  the United States," the Treasury secretary said. "It's better for it to  come from a multilateral context."
However, Geithner wouldn't say  whether he would be willing to talk to other countries about  coordinated action forcing nations that are being less flexible with  their currency to move forward.
Geithner said the most important policy challenge all global nations  face,  including the U.S., is figuring out how to improve economic  growth. 
"Most of us still have the capacity to take additional  actions that would improve both short-run and long-run growth  prospects," Geithner said. 
He suggested that the U.S.  policymakers also have more they could do but warned a big U.S.  challenge is a lack of political will, implying that the problem lies  with Congress.
"The most important thing we can do now is to make  sure this recovery is self-sustaining," he said. "Then we'll be in a  much better position to address fiscal challenge," like growing budget  deficits.
Geithner said that the U.S. economy is "absolutely  healing," in spite of sluggish economic reports this summer. He  acknowledged continued weakness in real estate and construction but  pointed to strength in the exports and the technology sector.
"It's  healing more quickly than any of us thought," Geithner said