Some 900 people have fled the Iraqi city of Mosul and crossed the border into Syria, the UN refugee agency says.
This is the first large group of civilians confirmed to have escaped since the Iraqi government began its offensive to liberate Mosul from the so-called Islamic State (IS).
Up to 1.5 million are thought to be in Mosul, with up to 5,000 IS fighters.
There are fears the militants will use the civilians as human shields as Iraqi forces get closer to Mosul.
A spokeswoman for the Office of UN High Commissioner for Refugees (UNHCR) said that more than 900 people from Mosul had crossed the border into Syria and were now at a refugee camp.
She said it was likely the agency would use the camp as a staging-post before moving them back across the border to a safe location in Iraq, the BBC’s Richard Galpin reports.
The movement of a significant number of people indicates that IS militants are not able to stop everyone leaving, our correspondent notes.
It will also raise questions about whether some fighters might try to use the same route to flee, he adds.
Mosul residents had been told by the Iraqi government that it may be safer to stay in the city while the operation is under way – with fears IS fighters have booby-trapped roads and placed snipers on routes in and out.
But there are also fears that IS militants could use residents as human shields by moving into their neighbourhoods, and concern that the group may even use chemical weapons.
Residents confirmed that IS was preventing people from fleeing the city and had directed some of them towards buildings likely to be targeted by air strikes.
US President Barack Obama said that it was important to ensure that Mosul’s residents could safely flee the city.
“If we aren’t successful in helping ordinary people as they’re fleeing Isil (IS), then that makes us vulnerable to seeing Isil return,” he told reporters in Washington.
The UN is working to create new refugee camps to the south, east and north of Mosul, and predict some 200,000 people will need shelter in the first days and weeks of the operation.
But many of those who have already escaped have headed west to camps across the border in Syria, including Al Hol camp.
Al Hol was built to house 7,500 people but currently has 9,000 refugees.
Some 912 people have arrived from Mosul in the last 24 hours, and 3,000 people have been dropped off by smugglers and are expected to be brought to Al Hol once they have been cleared by security, the UN says.
While Al Hol is being expanded to eventually take in 50,000 people, Save the Children says the camp currently has just 16 latrines, is littered with waste and faeces, and has no clean water.
“These families arrive with nothing but the clothes on their backs and find almost nothing to help them,” said the charity’s Tarik Kadir. “The camp is bursting at the seams and risks being overwhelmed.”
Iraqi troops are moving towards Mosul from the south, while their Kurdish allies have been approaching from the east.
They have taken back control of some 10 villages and are now between 30 and 40 km (19 and 25 miles) from the city.
But progress is expected to be slow, and a Kurdish commander Sirwan Barzani that it could be two weeks before Iraqi troops enter Mosul and two months before the city is liberated.
IS fighters appear to be putting up stiff resistance in places, with the group releasing video purporting to show them firing on coalition armored vehicles.
Mosul is the oil-rich capital of Nineveh province and Iraq’s second-largest city.
It was overrun by IS in June 2014 and was the place from where IS leader Abu Bakr al-Baghdadi proclaimed a “caliphate” in parts of Iraq and neighboring Syria.