ZURICH -- Shares in Switzerland's two largest banks Credit Suisse Group (CS) and UBS AG (UBS) plunged Thursday after the Swiss National Bank said they must keep strengthening their capital base.

Jean-Pierre Danthine, vice chairman of the SNB's governing board told a press conference that large Swiss banks needed to boost their resilience.

At 0845 GMT, shares in Credit Suisse was down 8% at CHF17.49 and UBS was down 2.2% at CHF10.92

"As soon as the SNB asked for more equity at the banks, the shares started to decline," said a Zurich-based trader.

-Write to John Revill at john.revill@dowjones.com