Switzerland's central bank and financial regulator on Wednesday issued tighter liquidity rules for UBS and Credit Suisse, part of a raft of measures aimed at the Swiss giants' role as "systemically important" to the Swiss economy.

The main element of the new rules is a "stringent" stress scenario which covers a general financial market crisis as well as creditors' loss of trust in a bank. Both Zurich-based banks will be required to comply with the new liquidity rules as of June 30.