A Kurdish-led force attacked Turkish-backed opposition fighters in northern Syria on Monday, killing at least 13 of the militants, activists said.

The opposition activists blamed the Kurdish-led Syrian Democratic Forces for carrying out the assault, though the U.S.-backed group did not claim responsibility.

Turkey says Syria’s main Kurdish militia is allied with the outlawed Kurdistan Workers’ Party, or PKK, that has led an insurgency against Turkey since 1984 that has killed tens of thousands of people.

Turkey since 2016 has conducted three major incursions into northern Syria to clear areas under Kurdish control and create a buffer zone near its border. Since then, the two groups have routinely clashed, while Turkey has also conducted airstrikes and drone attacks on targets in Kurdish-controlled areas.

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According to opposition activists, SDF forces tried to infiltrate the opposition-controlled city of Tal Battal in northern Aleppo province, attacking positions belonging to Turkish-backed militants and the al-Qaida-linked Hayat Tahrir al Sham.

Meanwhile, the Britain-based opposition war monitor Syrian Observatory for Human Rights, said the death toll was at 14. According to the Observatory, mines exploded during the attack that took place at dawn.

The SDF has been the main U.S. ally in Syria in the campaign against the Islamic State group that was defeated on the battlefield in the war-torn country in March 2019.

The U.S. has some 900 troops in eastern Syria backing SDF forces in targeting militant Islamic State group sleeper cells.