ISTANBUL — Turkish artillery fired shells into the northern Syrian region of Afrin on Friday as Turkey’s defense minister announced a military operation against American-backed Kurdish militias, despite warnings from the United States. The developments threatened to further destabilize a region fractured by seven years of war.

Fighters of the Turkish-backed Free Syrian Army, which were expected to spearhead the attack, were seen driving to the border around midday as artillery sounded on Turkey’s southern border with Syria. The Syrian fighters posted videos of men and vehicles preparing for the assault.

Turkey has said the Kurdish militias are part of a terrorist group that threatens its security, but it has struggled to win support for an intervention against them. Syria opposes Turkey’s intervention, and Syria’s deputy foreign minister, Faisal Mekdad, said the Syrian Air Force would destroy any Turkish airplanes used in an assault on Afrin.

Turkey’s defense minister, Nurettin Canikli, announced the Afrin operation in a live studio interview on the pro-government A Haber television network on Friday afternoon.