"Turkey has responded in this manner in the past," said Turkey's Deputy Prime Minister Yalcin Akdogan. "The YPG crossing west of the Euphrates is Turkey's red line."
The YPG is the main fighting force of Syrian Kurds and a key ally of the U.S.-led coalition battling the Islamic State group. Turkey, which is also in the alliance, considers it an affiliate of the Kurdistan Workers' Party, or PKK, which has waged a decades-long insurgency against Ankara.
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