By Susan Cornwell

WASHINGTON, April 22 (Reuters) - President Barack Obama will have some persuading to do to get the U.S. Congress to approve more money for the International Monetary Fund, House Speaker Nancy Pelosi said on Wednesday.

Pelosi suggested part of the challenge was uncertainty about how to count the cost of the additional resources for the IMF, at a time when U.S. bailout fatigue and concern about mounting U.S. debt are widespread on Capitol Hill.

"The president has requested from us $100 billion for the IMF and that's going to take its level of persuasion as well," Pelosi said during a roundtable for reporters sponsored by the Christian Science Monitor.

Obama wrote to Pelosi and other congressional leaders on Monday, urging Congress to quickly pass legislation to allow the United States to keep promises he made at the Group of 20 nations meeting in London this month.

Those promises included a U.S. pledge of $100 billion for the IMF to help it combat the global economic crisis. The funding would boost the IMF's "New Arrangements to Borrow," under which countries like the United States provide credit to the fund to deal with severe crises that threaten the global financial system.

Obama's letter said the U.S. contribution would essentially be a loan to the IMF and would not cost U.S. taxpayers anything, apparently reflecting the judgment of the administration's Office of Management and Budget (OMB).

But Pelosi said some congressional scorekeepers who weigh the cost of proposals see it differently.

"I'm certain we will be able to achieve it, but just there, for example, the OMB is saying there's no scoring because there are no outlays and the CBO is saying it's $105 billion," Pelosi said, referring to the Congressional Budget Office.