ISTANBUL — The reports are like something out of a distant era of ancient conquests: entire villages emptied, with hundreds taken prisoner, others kept as slaves; the destruction of irreplaceable works of art; a tax on religious minorities, payable in gold.

A rampage reminiscent of Tamerlane or Genghis Khan, perhaps, but in reality, according to reports by residents, activist groups and the assailants themselves, a description of the modus operandi of the Islamic State’s self-declared caliphate this week. The militants have prosecuted a relentless campaign in Iraq and Syria against what have historically been religiously and ethnically diverse areas with traces of civilizations dating to ancient Mesopotamia.

The latest to face the militants’ onslaught are the Assyrian Christians of northeastern Syria, one of the world’s oldest Christian communities, some speaking a modern version of Aramaic, the language of Jesus.

Assyrian leaders have counted 287 people taken captive, including 30 children and several dozen women, along with civilian men and fighters from Christian militias, said Dawoud Dawoud, an Assyrian political activist who had just toured the area, in the vicinity of the Syrian city of Qamishli. Thirty villages have been emptied, he said.