In the darkness of Sunday morning, the Sunni fighters swept in to take one of the towns, Sinjar, and set about their method of conquest, which is as familiar as it is brutal: They destroyed a Shiite shrine, executed resisters, overran local security forces and hoisted the black flag of the Islamic State in Iraq and Syria, or ISIS, above government buildings.
Hours later, as the militants demanded that the city’s residents swear allegiance to ISIS or be killed, the group’s social media campaign was underway, with photos posted online showing militants patrolling the city’s streets.
The United Nations representative in Baghdad, Nickolay Mladenov, issued a statement on Sunday afternoon, citing reports he had that as many as 200,000 civilians, mostly from the minority Yazidi community, had fled the new fighting.
“A humanitarian tragedy is unfolding in Sinjar,” Mr. Mladenov said.
In the face of stiff resistance from Shiite militias aligned with Iran that have stalled their march on Baghdad, the ISIS fighters who captured Mosul in June pushed north during the weekend. By Sunday afternoon, they were in control of two other towns after fierce battles with Kurdish security forces, known as the pesh merga, who have been increasingly thrust into battle to defend the border of their autonomous region in northern Iraq from encroachments by ISIS.
In a statement, ISIS boasted of conquering “more important areas which were controlled by the pesh merga and the secular militias.” With the new territory, which the group described as “the border triangle of Iraq, Syria and Turkey,” ISIS strengthened its hold on territory that traverses the frontiers of Iraq and Syria, giving it an even greater ability to move fighters and weapons between the front lines of the civil wars in both countries.
Later on Sunday, the militants captured Wana, a strategic town near the Tigris River — putting them within striking distance of the Mosul Dam, the country’s largest and an important supplier of electricity and water. The dam is on the Tigris River about 30 miles northwest of Mosul, Iraq’s second-largest city, which fell to ISIS on June 10.
Yazidi residents of Sinjar, who were reached by phone, were terrified. They told of kidnappings and executions of members of their sect. One resident, Sami Hassan, said he was at work as a nurse at a hospital on Sunday when an injured ISIS fighter arrived and demanded to know the sect to which Mr. Hassan belonged.