Fox News reports:

U.S. and Russian troops in the areas of the attacks were warned about an hour before 24 Turkish jets and two Turkish drones bombed locations in northern Iraq and northeast Syria. Senior Russian officials contacted U.S. counterparts to try to get Turkey to back off the strikes, but those pleas were ignored by the Turks, officials said.

…

The U.S. views the Syrian-Kurdish People’s Protection Units, known as the YPG, as key allies in the fight against ISIS. Turkey views the group as an extension of the Kurdistan Worker’s Party, or PKK, a Turkish insurgent group that has conducted a decades long insurgency that’s killed thousands. Both Ankara and Washington consider the PKK a terrorist organization.

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