U.S. lawmakers including Senator Elizabeth Warren and Democratic presidential candidate Bernie Sanders on Thursday sent a letter to Federal Reserve Chair Janet Yellen urging more diversity at the U.S. central bank.

Ten of the Fed's 12 regional bank presidents are men; 11 of them are white, the letter noted.

"Given the critical linkage between monetary policy and the experiences of hardworking Americans, the importance of ensuring that such positions are filled by persons that reflect and represent the interests of our diverse country cannot be understated," said the letter, signed by 116 members of Congress and 11 Senators.

The Fed has come under fire in recent months from both Republicans and Democrats, including candidates for the 2016 presidential campaign, for a range of perceived failings, from its process to deciding monetary policy to its governance. Those calls have emboldened lawmakers who seek to limit the Fed's powers and are prompting some current and former Fed officials to call for steps to placate the bank's harshest critics.

In 2013, African-Americans made up about 13 percent of the 384 executives and top managers at the Fed's Washington-based Board of Governors, while Hispanics comprised just under 3 percent, according to figures provided by the Fed.

Of the 1,641 employees classified as professionals, blacks made up nearly 17 percent and Hispanics about 5 percent.