President Recep Tayyip Erdogan said the operation against the Afrin enclave aims to "purge terror" from his country's southern border.
Afrin is controlled by a Syrian Kurdish militia known as the YPG. Turkey considers the YPG to be a terrorist group linked to the outlawed Kurdistan Workers' Party (PKK) that has waged a bloody insurgency within its borders.
A YPG spokesman in Afrin said clashes erupted after midnight between his unit and Turkish troops near the border with Turkey. Rojhat Roj said the shelling of areas in Afrin district, in Aleppo province, killed one YPG fighter and injured a couple of civilians on Sunday.
Turkey and its Western allies, including the U.S., consider the PKK a terrorist organization. But the U.S. has been arming some of Syria's Kurds to defeat the Islamic State group in Syria - a sore point in already tense U.S.-Turkish relations.
The Turkish president said "despite it all" he wants to work with the U.S. in the region and hopes it will not side with the YPG during the upcoming Afrin operation.
"We expect (the U.S.) to support Turkey in its legitimate efforts" to combat terror, said Erdogan.
Also Sunday, Erdogan's spokesman responded to reports the U.S.-led coalition would establish a 30,000-strong border security force in Syria involving the Kurdish militia as "worrying."
In December, The Associated Press reported that the U.S. was developing an expanded training program for Kurdish and Arab border guards in Syria to prevent the resurgence of IS.
Ibrahim Kalin, the presidential spokesman, said the U.S. was taking steps to legitimize and solidify the YPG. "It's absolutely not possible to accept this," Kalin said and repeated that Turkey would defend itself.
Erdogan said the new operation into Afrin would be an extension of Turkey's 2016 incursion into northern Syria, which aimed to combat IS and stem the advance of U.S.-backed Kurdish forces. Turkish troops are stationed in rebel-held territory on both sides of Afrin.
Roj said the Kurdish militia will fight to "defend our gains, our territories." Senior Kurdish official Hediye Yusuf wrote on Twitter that the Turkish operation against Afrin is a "violation" of the Syrian people and undermines international efforts to reach a political solution in Syria.
The Turkey-PKK conflict has killed an estimated 40,000 people since 1984 and the resumption of hostilities in July 2015 killed more than 3,300 people, including state security forces, militants and civilians.
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