Updated

Hundreds of Iraqi men, women and children crammed into vehicles fled their homes Thursday, fearing clashes, kidnapping and rape after Islamic militants seized large swathes of northern Iraq.

The families and fleeing soldiers who arrived at a checkpoint at the northern frontier of this largely autonomous Kurdish region in Iraq were among some half-million people who have fled their homes since Monday, according to a U.N. estimate.

Workers were busily extending the Khazer checkpoint in the frontier area known as Kalak, where displaced women hungrily munched on sandwiches distributed by aid workers and soldiers rushed to process people.

The exodus began after fighters of the al-Qaida breakaway group, the Islamic State of Iraq and the Levant, seized the northern city of Mosul in a stunning assault Monday. Since then, the militants have moved southward toward the capital, Baghdad, in the biggest crisis to face Iraq in years.

"Masked men came to our house and they threatened us: 'We will get to you.' So we fled," said Abed, a laborer who abandoned his home on the edge of Mosul Thursday. "They kidnapped other people. They took away some people for interrogation."

The young man said rumors were quickly spreading that Islamic State fighters — as well as masked bandits taking advantage of the chaos — were seizing young women for rape or forced marriage.

"They are destroying the honor of families," said Abed, who, like many of the displaced, wouldn't give his full name, fearing the Islamic State fighters.

Many of the displaced said they were on the move because they feared retribution by Iraq's military — underscoring the grave sectarian tensions that have allowed the Islamic State fighters, who are Sunni extremists, to conquer so fast and deeply.

Mosul, Iraq's second-largest city, is mostly Sunni, and many residents have long complained of discrimination and mistreatment by the Shiite-dominated central government.

"We were worried the struggle would get bigger, that Maliki's army would shell us," said a middle-aged Sunni woman, referring to the country's Shiite prime minister, Nouri al-Maliki.

"Whoever will rule us — let them rule us," said her husband Talal Ahmad, 62. "We just want our children to be safe."

Many waiting to be processed at the Khazer checkpoint, set among golden wheat fields, echoed similar concerns. Most hadn't seen fighting but heard occasional gunshots. They saw other people fleeing and so joined the exodus.

Many said they panicked after hearing Iraqi army soldiers had abandoned their posts, sure it meant that heavy shelling to drive out the insurgents would follow.

"We left after we saw everybody else leaving," said Abir, a 33-year-old teacher who fled with her husband and three children.

The chaos of the fighting, just some 60 miles away, was evident in Kalak.

Kurdish forces, which act as a de-facto military in the largely autonomous region, took possession of at least a dozen Iraqi military vehicles abandoned by soldiers as they fled their posts ahead of the advancing Islamic State fighters.

The Kurdish soldiers could be seen driving the dirty yellow Humvees, with the national flag emblazoned on them, toward the regional capital, Irbil.

One fleeing Iraqi soldier said he was ordered by his officer to abandon his post, even before Islamic State fighters reached the area.

"We didn't even raise our weapons. This isn't even unimaginable — it's madness," said 38-year-old Shaker Karam. "We didn't even see a terrorist."

At the checkpoint, Kurdish workers erected shelters in anticipation of the arrival of more displaced Iraqis.

Four men measured out an area amid a whipping dust and rain storm to protect the long lines of Iraqis from the sweltering heat. Beside them lay a large pile of water bottles to distribute. Just hours before, they set up a row of public toilets and erected a tent for exhausted women to rest in privacy.

Those who reached the Khazer checkpoint were among the lucky ones.

The U.N. children's agency, UNICEF, said thousands of displaced, particularly children, were sheltering in schools, hospitals and mosques outside Mosul, many of them without adequate water, sanitation, or shelter. The Red Cross said it had already distributed food and relief to 8,000 people near Mosul.

Many fled with little more than the clothing on their backs and, arriving without money said they would have to rely on donations.

Abed's extended family, including his elderly mother and young nieces, said they didn't know where they would sleep Thursday night.

Talal Ahmad's family of 12 was sleeping in the back of a pickup truck that was lined with thin mattresses.

Abir, the teacher, said her middle-class family had enough money for a hotel for a month.

"But we hope to be back before then," she said anxiously.