LONDON — Tens of thousands of people are fleeing fighting in northeast Syria, according to the United Nations refugee agency, as world leaders warned that Turkey’s invasion could spark a new humanitarian crisis.

“Hundreds of thousands of civilians in northern Syria are now in harm’s way. Civilians and civilian infrastructure must not be a target,” U.N. High Commissioner for Refugees Filippo Grandi said in a statement Thursday.

Pictures and videos circulating across social media Thursday showed lines of cars streaming out of towns along the Turkey-Syria border. Footage appeared to show families crammed into vehicles with children in the arms of parents next to suitcases and other portable possessions.

The Syrian Observatory of Human Rights said it had documented the death of eight civilians, two of whom are children, due to Turkish ground shelling. NBC News could not independently verify this claim.

Turkey's invasion of northeastern Syria began Wednesday after U.S. troops pulled back from the area to clear the way for Turkish forces.

Turkish troops are fighting the Syrian Democratic Forces, which are led by the Kurdish People's Protection Units (YPG). Turkey views the YPG as an extension of the separatist Kurdistan Workers’ Party (PKK), which is considered a terrorist group by the United States.

But the Kurdish-led Syrian Democratic Forces (SDF) have been crucial U.S. allies in the war on the Islamic State militant group.

The Turkish Defense Ministry maintains it is only targeting PKK, YPG and ISIS terrorists, as well as their shelters, weapons and equipment.

“People are fleeing when they can in cars, if not in the back of trucks, if not on foot, with what they can carry, old people, families, the sick,” said Thomas McClure from the Rojava Information Center, a pro-SDF research group based in the Kurdish-held areas.

McClure who spoke to NBC News via WhatsApp said most people were fleeing south to other areas under the Kurdish Autonomous Administration, while a smaller number were trying to leave the country.

“There’s no way out, they’re not being able to cross into Turkey nor by the Iraqi government,” he said. NBC News could not confirm the group’s claims.

In a joint statement Thursday, 14 aid agencies warned that some 450,000 people live within 3 miles of the Syria-Turkey border, and that some 90,000 of them are already internally displaced meaning they have already had to leave their homes during Syria’s unrelenting war.

In a separate statement, Save the Children said thousands of children were fleeing hostilities along with their families overnight and confirmed that it would scale up its relief operations. The United Nations warned that those fleeing the fighting were more at risk because temperatures were falling as colder weather sets in.