In an interview with the Asahi Shimbun newspaper, Geithner was quoted as saying U.S. authorities were making sure there was steady funding and that banks were able to meet commitments.
"So in some ways what we're saying is we're going to backstop the amount of capital-raising that's necessary," he was quoted as saying in an English text of the interview.
"And again, a lot of that will come from the market, ultimately. But where it doesn't we'll make sure we provide it."
In an attempt to assess banks' capital needs, the U.S. government is testing how they would fare under more adverse economic conditions than are expected. The results are due at the end of April.
Once the "stress tests" are finalized and the capital needs are determined, banks will have six months to raise capital in the private market or could take an infusion of government funds.
Geithner was also quoted as saying the Group of Seven of rich nations and the Group of 20, which also includes emerging economies such as China and India, were "very complementary forums."