Despite quick action by regulators and policy makers, there's a rising risk that banking-system stress will spill over into other sectors and the U.S. economy, "unleashing greater financial and economic damage than we anticipated," said Moody's Investors Service, one of the Big Three credit-ratings firms.

Simply put, the risk is that officials "will be unable to curtail the current turmoil without longer-lasting and potentially severe repercussions within and beyond the banking sector," Atsi Sheth, Moody's managing director of credit strategy, and others wrote in a note distributed on Thursday. Still, the agency's baseline view is that U.S. officials will "broadly succeed."

Moody's warning came as Treasury Secretary Janet Yellen indicated that the U.S. could take additional actions if needed to stabilize the banking system, and after Federal Reserve Chairman Jerome Powell assured Americans on Wednesday that the central bank would use its tools to protect depositors.