The ancient northern Iraqi town of Sinjar emptied Sunday, with thousands of people fleeing on foot as Sunni extremist militants made their first significant punches through the defenses of overstretched Kurdish forces.

Sinjar is an ancestral home of the long-persecuted Yazidi religious sect, which the Islamic State has branded as devil worshipers, and few of its residents stayed to find out what was planned for them when the group’s militants entered Sunday.

Until Sunday, Sinjar had been protected by Kurdish fighters known as pesh merga, but officials from the semi-autonomous northern region have been warning for weeks that they are poorly equipped to sustain the defense of the nearly 650-mile border they now share with the militants.

The town of Wana also fell Sunday, putting the Islamic State within striking distance of Mosul’s hydroelectric dam, the largest in the country. After nearly two months of skirmishes, it was a second day of losses for the Kurds with extremists seizing the nearby town of Zumar on Saturday and two small oil fields.

The al-Qaeda splinter group marked its entry into Sinjar by blowing up a Shiite shrine and demanding that residents convert or die. Islamic State-affiliated social media accounts circulated pictures of its gains — one claimed to show abandoned military vehicles, another a gun-toting militant sitting at the governor’s desk.

The pesh merga are known for their fighting prowess, but rocky relations with Baghdad mean they have no munitions supplies from the central government and their soldiers have not received salaries for months. Meanwhile, their lack of sovereignty complicates arms purchases from abroad and Baghdad put a temporary hold on cargo flights to the north last month.

The United States is “gravely concerned” by the displacement of innocent civilians and loss of life, said State department spokeswoman Jen Psaki. Joint Operations Centers in Irbil and Baghdad are sharing information with Iraqi security forces and Kurdish pesh merga, she said.

But Kurdish officials said they were poised to fight back, and Syrian Kurdish forces claimed to be assisting. Salih Muslim Mohammed, leader of the Kurdish Democratic Union Party, said that hundreds of fighters from his party’s armed wing are fighting alongside the pesh merga in Iraq. In a statement, the group’s military wing said it had been “surprised” by pesh merga withdrawals.

At least two Syrian Kurdish fighters were killed in Iraq, Mohammed said.

“This is not a time to dwell on political differences, it’s a time for working together,” he said, referring to past friction. “Our people are in danger and crisis. We have no choice but to assist.”

Including those in surrounding districts, as many as 200,000 were forced to flee in the new violence, said Nickolay Mladenov, the United Nations representative in Baghdad. He warned of an unfolding “humanitarian tragedy.”

A Kurdish official who spoke on the condition of anonymity said pesh merga forces had made a “tactical retreat” from Sinjar to allow civilians to leave after overnight fighting. Soldiers withdrawing from the town said that they had run out of ammunition, said Falah Hassan, 42, a Yazidi resident who spoke by phone as he fled across the mountains on Sunday.

Sinjar is an ethnically and religiously mixed town, and its population had swelled in recent weeks by those seeking refuge. Among them were thousands of Shiites and Yazidis from Tal Afar, which the Islamic State seized last month.

With the main road to safety passing through Tal Afar, Sinjar’s residents had few options when Islamic State forces drove into town Sunday, with many grabbing what they could and making for the mountains on foot.

“People were terrified,” said Ilias al-Hussani, 27, who had been trekking through the mountains for 10 hours. “They are savages. We’ve seen what they’ve done to people of their own faith. Imagine what they would do to us non-Muslims.”

Like most, he was heading for Dohuk in Iraqi Kurdistan, a five-hour drive.

“It’s miserable,” he said. “There are old women who can’t continue, and they are just sitting in the mountains. We’ve run out of water.”

Residents said they feared for the lives of the sick and elderly who had stayed behind, with reports that the Islamic State had begun to execute those who had refused to convert.

With its roots in Zoroastrianism, followers of the Yazidi faith have suffered persecution for centuries.

“In our history, we have suffered 72 massacres,” Yazidi parliamentarian Haji Ghandour said as he tried to leave the town on Saturday night, but was stuck on the road because of fighting. “We are worried Sinjar could be a 73rd.”

Mustafa Salim contributed to this report .