On Saturday, 100 or so women joined the list, turning up crammed into two buses at a school in the town of Tal Afar, where hundreds of the women are already being held, according to an eyewitness. The new arrivals had been detained the previous day in the small village of Kocho, where Kurdish officials and Yazidis say that more than 80 men were lined up and shot before the younger women were separated from the older ones and taken away.

The oldest of the women in Kocho were not detained but are being held there by Islamic State fighters who also spared the oldest men, said Ziad Sinjari, a Kurdish pesh merga commander in Sinjar, citing the account of one of six survivors of the massacre who escaped injured to a nearby village.

Once at the school, the eyewitness said, the youngest women again were parted from older ones and driven away, along with a dozen or so boys between ages 10 and 12 who had apparently been detained with their mothers.

The reports from the massacre at Kocho and its aftermath illustrate a disturbing pattern that has emerged in the two weeks since the majority-Yazidi town of Sinjar was overrun, prompting tens of thousands of panicked adherents of the minority sect to run for their lives to the mountains.