Yellen's call for more bank mergers causes regional bank stocks to tumble.

Shares of US regional banks fell after Treasury Secretary Janet Yellen told bank CEOs that more mergers may be necessary following a series of bank failures. The KBW Regional Banking Index fell 2.2%, with PacWest Bancorp and Western Alliance among the biggest losers. The regional bank crisis has been partly blamed on aggressive interest rates by the US Federal Reserve. Fed Chairman Jerome Powell said the after-effect of recent banking sector troubles is expected to take some pressure off the US central bank's interest rate hiking cycle.