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.