Turkish President Tayyip Erdogan greets people as he arrives at the site of Saturday's blasts, in Istanbul, Turkey, December 12, 2016. REUTERS/Yagiz Karahan

ANKARA (Reuters) - Turkey is facing the biggest attack in its history from terrorist groups and needs to retaliate in kind, President Tayyip Erdogan said in a speech to local administrators in the presidential palace on Wednesday.

Turkey has been hit by a series of bombings blamed on Islamic State and Kurdish militants this year and is pursuing a crackdown on the followers of U.S.-based cleric Fethullah Gulen, whom it blames for a failed coup attempt in July.