Turkish President Tayyip Erdogan addresses his supporters at the Presidential Palace in Ankara, Turkey, April 17, 2017. REUTERS/Umit Bektas

ANKARA (Reuters) - U.S. President Donald Trump called Turkish counterpart Tayyip Erdogan to congratulate him on winning a referendum expanding his authority, sources in Erdogan’s palace said on Monday.

Turkey voted on Sunday to switch to a presidential system, greatly increasing Erdogan’s powers. Unofficial results, which the opposition said it would challenge, showed a narrow victory for him with 51.4 percent of votes cast in favor.