BEIRUT, Lebanon -- Turkey-backed Syrian 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, mostly civilians, according to rebels and a monitoring group.
Turkey sent tanks across the border to help Syrian rebels drive the Islamic State group out of the frontier town of Jarablus last week in a dramatic escalation of its involvement in the Syrian civil war.
The operation, labeled Euphrates Shield, is also aimed at pushing back U.S.-allied Kurdish forces. The fighting pits a NATO ally against a U.S.-backed proxy that is the most effective ground force battling IS in Syria.
Turkish officials could not immediately be reached for comment.
A Turkish soldier was killed by a Kurdish rocket attack late Saturday, the first such fatality in the offensive, now in its fifth day.
Various factions of the Turkey-backed Syrian rebels said 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 U.S.-backed Syria Democratic Forces, viewing it as an extension of the Kurdish insurgency raging in southeastern Turkey. Turkish leaders have vowed to drive both IS and the Kurdish People's Protection Units, or YPG, away from the border.
The SDF crossed the Euphrates River and drove IS out of Manbij, a key supply hub just south of Jarablus, earlier this month. Both Turkey and the United States have ordered the YPG to withdraw to the east bank of the river. YPG leaders say they have, but their units play an advisory role to the SDF and it is not clear if any of their forces remain west of the Euphrates.
Turkey is part of the U.S.-led coalition fighting IS, but the airstrikes that began 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 (15 kilometers) south of Jarablus, and another 15 in a village to the west.
ANHA, the news agency of the Kurdish semi-autonomous areas, said Beir Khoussa has "reportedly lost all its residents."
SDF spokesman Shervan Darwish said the airstrikes and shelling started overnight and continued Sunday along the front line, killing many civilians in Beir Khoussa and nearby areas. He said the bombing also targeted Amarneh village. He said 50 Turkish tanks were taking part in the offensive.
Syrian state news agency SANA reported that 20 civilians were killed and 50 wounded in Turkish artillery shelling and airstrikes, calling it Turkish "encroachment" on Syrian sovereignty under the pretext of fighting IS. Turkey is a leading backer of the rebels fighting to overthrow Syrian President Bashar Assad.
An Associated Press reporter in the Turkish border town of Karkamis spotted at least three Turkish jets flying into Syria amid heavy Turkish shelling from inside Syrian territory on Sunday morning.
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