A large U.S. investment bank is considering building up a 10% stake in Italy's Banca Popolare di Sondrio, Il Sole 24 Ore newspaper reported on Thursday, without citing sources or naming the investor. The move would also pave the way for the entry of UniCredit in the group's shareholding, the newspaper said, without giving further details and adding that UniCredit declined to comment. Banca Popolare di Sondrio declined to comment on the report.