Turkey-backed rebels seized a number of villages and towns from Kurdish-led forces in northern Syria on Sunday amid Turkish airstrikes and shelling that killed at least 35 people, according to a monitoring group.
Turkey sent tanks across the border to help Syrian rebels drive the Islamic State (IS) group out of the frontier town of Jarablus last week in an escalation of its involvement in the Syrian civil war.
The operation - labelled Euphrates Shield - is also aimed at pushing back US-allied Kurdish forces.
The fighting pits a Nato ally against a US-backed proxy that is the most effective ground force battling IS in Syria.
A Turkish soldier was killed by a Kurdish rocket attack late on Saturday, the first such fatality in the offensive, now in its fifth day.
Various factions of the Turkey-backed Syrian rebels said on Sunday they have seized at least four villages and one town from Kurdish-led forces south of Jarablus.
One of the villages to change hands was Amarneh, where clashes had been fiercest. Rebels posted pictures from inside the village.
Ankara is deeply suspicious of the Syrian Kurdish militia that dominates the US-backed Syria Democratic Forces, viewing it as an extension of the Kurdish insurgency raging in south-eastern Turkey.
Turkish leaders have vowed to drive both IS and the Kurdish People's Protection Units, or YPG, away from the border.
Turkey is part of the US-led coalition fighting IS but the airstrikes that began on Saturday marked the first time it has targeted Kurdish-led forces in Syria.
The Britain-based Syrian Observatory for Human Rights said the bombing killed at least 20 civilians and four Kurdish-led fighters in Beir Khoussa, a village about nine miles south of Jarablus, and another 15 in a village to the west.