Federal Reserve Vice Chair Randal Quarles emphasized there is "no path forward" for Libor, which is being scrapped after numerous banks were fined for manipulating it, and that firms have no reason to delay moving derivatives and other market contracts to the new Secured Overnight Financing Rate.

"The deniers and laggards are engaging in magical thinking," Quarles said during a meeting of the Financial Stability Oversight Council, a regulatory panel. "Libor is over."

A host of senior U.S. officials, including Treasury Secretary Janet Yellen and Fed Chair Jerome Powell, echoed that message, as regulators worry that financial firms are moving away too slowly from the previous benchmark, which is set to expire at the end of this year for new contracts.

Yellen said some sectors, including business loans, are "well behind" where they should be in the transition. Regulators have struggled for months to convince market participants to abandon Libor, and have relied on increasingly severe rhetoric to convince them that the benchmark will no longer be an option in the near future.

At the same time, regulators had harsh words for competing benchmarks, like the Bloomberg Short-Term Bank Yield Index (BSBY). They warned that, like Libor, those benchmarks are built on relatively few transactions, which could make them unreliable or subject to manipulation.

Securities and Exchange Commission Chairman Gary Gensler said BSBY is built around less than $10 billion in transactions per day when it is meant to serve as a foundation for trillions of dollars in transactions.

"When a benchmark is mismatched like that, there's a heck of an economic incentive to manipulate it," he warned.

(Reporting by Pete Schroeder; Editing by Leslie Adler and Paul Simao)

By Pete Schroeder and Katanga Johnson