"This is the first time that Turkmen fighters have led the battle against IS in the area, and that comes after they received support from Turkey," said Rami Abdel Rahman, director of the Syrian Observatory for Human Rights monitor.
He did not provide details on the alleged assistance.
The Britain-based group said the 13 fighters killed since Friday were members of the Sultan Murad Brigades, made up of Turkmen fighters, battling IS in the northern province of Aleppo.
The clashes were centred on a region to the east of Azaz, a town on the Turkish border, that is home to many of Syria's Turkmen minority, it said.
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It added that the Turkmen had captured three border villages.
The Observatory did not have a casualty toll for the jihadists but said one of their vehicles was destroyed in an air strike by the US-led coalition in the Azaz area Friday night.
IS controls large stretches of the border area, but Ankara and Washington have sought to ensure the group is cleared from the area between Jarabulus and Azaz.
Turkey sees Syria's Turkmen in Syria as natural allies and reports have frequently surfaced that Ankara wants them to lead the fight against IS in areas near the border.
That is sensitive because the most effective Syrian force fighting IS has so far been the Kurdish People's Protection Units (YPG), which have been backed by the US-led coalition.
Ankara considers the YPG to be a branch of the Kurdistan Workers Party, which it deems a "terrorist" organisation.