ST. GALLEN, Switzerland, May 6 (Reuters) - UBS CEO Sergio Ermotti on Wednesday said the Swiss bank needs to broaden its business in the United States as he pledged to prepare for any scenario that could emerge from a parliamentary debate on proposed tougher capital rules for the lender.

Speaking in the Swiss city of St. Gallen, Ermotti vowed to work until the "last minute" to ensure UBS and Switzerland kept benefiting from each other amid speculation the bank could move its headquarters abroad if it found the new rules too onerous.