Turkish tanks shelled a village held by Kurdish forces in northern Syria overnight, wounding at least four allied fighters, a monitoring group and activists said.
"A number of shells fired by Turkish tanks fell on the village of Zur Maghar, which is controlled by the Kurdish People's Protection Units (YPG)," the Syrian Observatory for Human Rights said.
"Four fighters were injured," the group added.
Observatory director Rami Abdel Rahman said the four were members of a non-YPG rebel force fighting alongside the Kurds against the Islamic State (ISIS) group.
Zur Maghar lies along the border with Turkey, east of the town of Jarabulus, in Aleppo province.
Abdel Rahman said the incident appeared to be the most serious Turkish targeting of Kurdish-controlled areas in the Syrian conflict.
Activists said there had been cross-border fire before but that the overnight incident was particularly serious because of the context.
In recent days, Turkey has begun striking ISIS in Syria and arresting the group's sympathizers at home, but it has also targeted the Kurdistan Worker's Party (PKK), a Kurdish group with strong links to the YPG in Syria.
Turkey considers the PKK a "terrorist organization," and in recent days has hit the group's positions in northern Iraq with the heaviest air strikes since August 2011.
"This shelling comes after Turkey declared war on Daesh (ISIS) and a war against the PKK," Syrian Kurdish activist Mustafa Ebdi said. "Now the YPG is facing attacks from ISIS and Turkey."
The YPG has proved Syria's most effective force against ISIS, but its successes have been eyed suspiciously by Turkey because of its links to the PKK.
Turkey has also been pressing the US not to arms the Kurds in their fight against ISIS.
AFP contributed to this report.