Treasury Secretary Timothy F. Geithner said the U.S. was “very forceful from the beginning” in expressing concerns about manipulation of the London interbank offered rate.

“We acted very early in response to concerns that the process that set this rate was impaired and flawed, and vulnerable to misrepresentation,” Geithner said at a conference in New York today. Geithner said he briefed U.S. financial regulators and wrote “a detailed set of recommendations” to British regulators.

Regulators in both the U.S. and U.K. defended their reaction to the manipulation of Libor, the global benchmark for $500 trillion of securities, after Barclays Plc was fined a record 290 million pounds ($453 million) for rigging borrowing costs.

The New York Fed last week released documents showing it knew Barclays underreported rates and that Geithner, then the president of the regional Fed bank, sent a memo in June 2008 to Bank of England Governor Mervyn King recommending changes to how Libor was calculated.

Geithner was speaking at the CNBC Institutional Investor Delivering Alpha Conference.

To contact the reporters on this story: Ian Katz in Washington at ikatz2@bloomberg.net; Meera Louis in New York at mlouis1@bloomberg.net

To contact the editor responsible for this story: Chris Wellisz at cwellisz@bloomberg.net