LONDON (Reuters) - Portuguese bond yields rose sharply on Thursday, while Italian and Spanish yields reversed an earlier fall after a Greek government official said Prime Minister Alexis Tsipras would resign later in the day. The move will clear the way for early elections on Sept. 20. Italian 10-year yields were up 2 basis points at 1.83 percent, having traded as low as 1.77 percent earlier. Spanish yields were flat at 2.0 percent, but up from the day's lows of 1.94 percent. Portuguese yields rose 12 basis points to 2.62 percent. Greek two-year bond yields rose 78 bps to 12.15 percent. ...