(Reuters) - A U.S. senator on Wednesday asked the New York Federal Reserve to explain further the actions it took to address possible manipulation by big banks of the Libor benchmark interest rate.

Democrat Sherrod Brown, who sits on the Senate committee that oversees the Fed, questioned Federal Reserve Chairman Ben Bernanke's assertions that he had no authority to address rigging problems with the key inter-bank lending rate.

In light of the Fed's past claims to strong expertise and authority to oversee the largest U.S. banks, "it is difficult to accept the argument that the Fed has no authority to address this problem," Brown said in a letter Wednesday to the New York Fed's president and director of bank supervision.

The Fed oversees large banks, including the U.S. branches of foreign banks such as Barclays Plc, which paid a record fine to settle charges that it manipulated the London Interbank Offered Rate.

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