Turkey's military said Wednesday its forces destroyed Syrian Kurdish targets overnight in retaliation for artillery fire against Turkish-backed forces in northern Syria.
A Turkish army statement said the clashes happened in the Afrin area.
The Kurdish People's Protection Units, or YPG, are a key part of the U.S.-backed Syrian Democratic Forces and in particular the ongoing fight to defeat Islamic State militants in the group's de facto capital of Raqqa.
But Turkey strongly opposes the Kurdish fighters, seeing them as linked to the outlawed Kurdistan Workers' Party, or PKK, that has been waging an insurgency in southeastern Turkey since the 1980s.
U.S. support for the YPG includes weapons deliveries that Turkey fears will end up in the hands of PKK fighters.
The Turkish government has said U.S. Defense Secretary Jim Mattis gave reassurances that the United States would take back those weapons after Raqqa falls, but Mattis indicated Tuesday that U.S. support for the YPG would continue.
"We'll do what we can," Mattis told reporters traveling with him. But he said the weapons provided to the YPG will depend on what the next mission is.
"When they don't need certain things anymore, we'll replace those with something they do need," Mattis said.
The U.S. defense chief is due to meet Thursday with his Turkish counterpart during a NATO meeting in Brussels.