In this image made from video posted online by Qasioun News Agency, a member of a Turkish-backed Syrian opposition force patrols in Dabiq, Syria, Sunday, Oct. 16, 2016.

A Turkish soldier was killed and five wounded in clashes with ISIS in northern Syria, the military said on Friday, as Turkish-backed rebels pressed an offensive to take the city of al-Bab from the radical militantsgroup.

The latest casualties bring the number of Turkish soldiers killed in Syria to 17 since Ankara launched a cross-border incursion on Aug. 24 to try to push ISIS and Kurdish fighters from the border, according to Turkish media.

The Turkish military said four Syrian rebels had been killed and 25 wounded in clashes in the 24 hours to Friday morning. Turkish fighter jets were continuing to strike ISIS targets near al-Bab, it said.

The casualties came a day after three Turkish soldiers were killed in a suspected Syrian air strike, the first such Turkish deaths at the hands of Syrian government forces. Prime Minister Binali Yildirim said Ankara would retaliate.

The advance by the largely Turkmen and Arab rebels towards al-Bab, the last urban stronghold of ISIS in the northern Aleppo countryside, potentially pits them against Kurdish fighters and Syrian government forces.

Al-Bab is of particular strategic importance to Turkey because Kurdish-dominated militias have also been pursuing a drive to seize it. Ankara is determined to prevent Kurdish forces from joining up cantons they control along the Turkish border, for fear it will stoke Kurdish separatism at home.

Turkey is backing the Syrian rebels with troops, tanks and artillery fired from within Turkey, as well as reconnaissance flights along the border, the military statement said. Washington has said the US-led coalition is not providing support for the operation.

President Tayyip Erdogan said last week that the Turkish-backed rebels were close to taking al-Bab despite some resistance and that they would then move to secure Manbij, a town to the east which was taken by Kurdish-dominated militia fighters from ISIS in August.

Forces allied to Syrian President Bashar al-Assad warned Turkey last month against making any advance towards their positions to the north and east of Aleppo, saying any such move would be met “decisively and with force”.

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