Turkey's military says it has begun attacking Kurdish-held positions in Syria's Afrin region, including reportedly firing on positions held by a group that the USA has backed in the past.
Officials in Ankara said Turkish Foreign Minister Mevlut Cavusoglu has spoken with U.S. Secretary of State Rex Tillerson about the offensive but did not provide details about the conversation, the Associated Press reported.
The U.S. has repeatedly armed and trained the YPG, which it sees as a vital partner in its fight against ISIS.
US officials have urged Turkey to restrain its offensive against the Kurds in Syria and focus on destroying the remnants of ISIS instead.
Earlier in the day, Turkish media reported four rockets fired from Syria had hit the Turkish southern border town of Kilis, wounding one person and causing damage to houses there.
Turkish ground forces have pushed into northern Syria's Afrin province as Ankara steps up artillery attacks on a US-backed Kurdish militia it aims to sweep from its border.
The UK-based Syrian Observatory for Human Rights and a spokesman for the Kurdish forces that control the area said eight civilians were killed on Sunday in Turkish air raids in Afrin.
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On Saturday, Turkish President Recep Tayyip Erdogan said the operation in Afrin would be followed by a push in the northern town of Manbij, which the US-backed Kurdish forces captured from ISIL in 2016.
"In the planning and execution of the operation only terrorists and their shelters, positions and weapons are being targeted", the army has said.
The YPG has also accused Turkey of striking civilian districts and a camp for the displaced in Afrin. The assault, the source said, "is the latest move in Turkey's attacks on Syria's sovereignty".
But the campaign risks further increasing tensions with Turkey's North Atlantic Treaty Organisation ally Washington, which has supported the YPG in the fight against Islamic State (IS) jihadists and warned Ankara about distracting the focus from that fight.
France has called for an emergency meeting of the United Nations Security Council over the Turkish incursion, French Foreign Minister Jean-Yves Le Drian said yesterday.
Mattis said the communication took the form of a telephone call between high level Turkish and USA military officials.
Dozens of Kurds took the streets of Copenhagen on Saturday, demanding a stop to operation Olive Branch.