Syrian rebels backed by Turkish tanks and jets pushed deeper into territory held by Kurdish-led forces in northern Syria on Sunday, leaving some 35 civilians dead in two separate attacks, a monitoring group said.

The rebels and Turkish forces drove fighters linked to the Kurdish-led Democratic Forces of Syria (DFS) - the main US ally against Islamic State jihadists in Syria - from a number of villages, the Syrian Observatory for Human Rights said.

The villages captured included at least one on the south bank of the Sakhour river, bringing the Turkish-backed rebels closer to the strategic city of Minbij which the DFS captured from Islamic State earlier this month after a gruelling 10-week campaign.

Turkey's official Anadolu Agency said 25 Kurdish fighters were killed. Kurdish sources did not confirm the figure.

The Observatory said 35 civilians were killed Sunday in the Turkish campaign, which started on Wednesday when Turkish forces and allied Syrian rebels seized the border town of Jarabulus, apparently meeting little resistance from Islamic State jihadists who had held it since 2013. 

The deadliest attack occurred in the Kurdish-held village of Jub al-Koussa, leaving at least 20 civilians dead and 50 injured, the Britain-based Observatory said.

At least 15 civilians were killed in a second attack when Turkish jets targeted a farm to which families fled for safety from violence south of Jarabulus, the Observatory added.

At least 25 civilians were injured.

There was no official comment in Turkey. The Observatory quoted a rebel local council as accusing the Kurdish forces of having brought about the killings by using the slain civilians as human shields.

The reported attacks came a day after Turkey acknowledged its first death in Syria. The Kurdish-aligned forces claim to have destroyed two Turkish tanks and killed a number of soldiers.

Turkey says its operations in northern Syria are directed against both Islamic State and the Kurdish Democratic Union Party (PYD), the main political force behind the DFS - which it regards as the Syrian branch of Kurdish rebels operating on its soil.

Islamic State has controlled territory in northern Syria near the Turkish border since 2013.

The Kurds, backed by intensive US airstrikes, began to push the radical group back in 2014, taking most of the border land.

The Kurdish-led capture of Minbij upped the stakes. Seizing the city from Islamic State was intended to cut the jihadists' links to the Turkish border and the outside world, making it harder for them to bring fighters and supplies to their territories deeper inside Syria and Iraq.

But the move increased Turkish and rebel fears that the Kurds would strike west to the town of al-Bab and seek to form a corridor linking up their main territories in north-eastern Syria with the Efrin enclave near Aleppo.

Such a move, a long-held Kurdish ambition, would leave Turkey faced with a Kurdish entity along most of its border with Syria.

The rebels, meanwhile, suspect the Kurds of separatist ambitions and accuse them of collaborating with the regime of Syrian President Bashar al-Assad.

The US has said that the Kurds should move east of the Euphrates, leaving Minbij in the hands of local forces, and that it will not back them with air power if they moved further.

Turkey is a staunch backer of rebel groups fighting to oust al-Assad and has moved hundreds of fighters into the Jarabulus region to fight Islamic State and the Kurds.

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