The United States began pulling troops from northeast Syria in a major policy shift, opening the way for a Turkish attack on Kurdish-led forces long allied to Washington and handing Turkey responsibility for thousands of jihadi captives.
The United Nations currently delivers aid to 700,000 people in the densely-populated northeast region of 1.7 million.
It has drawn up contingency plans to reach people who might flee south with food and medical aid, said Panos Moumtzis, U.N. regional humanitarian coordinator for the Syria crisis.
"Any (military) operation that takes place at the moment has to take into account to ensure that we don't see any further displacement," Moumtzis told reporters in Geneva. "We are hoping for the best but preparing for the worst".
Turkey has long argued for the establishment of a 20-mile (32 km) "safe zone" along the border, under Turkish control, driving back the Syrian Kurdish YPG militia - which is the dominant force in the SDF alliance and which Ankara considers a terrorist organisation and a threat to its national security.
Moumtzis, asked about the Turkish offensive against Kurdish-led forces and the 'safe zone' idea, said:
"I really cannot talk about what type or how would the intervention be or how extensive it is going to be in terms of the military operation that will take place.
"For us as the United Nations, the safe zone concept is one that we have a bitter history (with) and actually we never promote or encourage. We don't think it is something that had worked for the United Nations, keeping in mind Srebrenica and what had happened in the past," he said.
He was referring to the slaughter by Bosnian Serb troops of 8,000 Muslim men and boys in 1995 in a U.N.-declared "safe zone" where Dutch peacekeepers were unable to protect civilians.
"We understand that there is going to be some kind of security zone which will be very specifically targeted to a military operation or to an area where there has to be some clearance," Moumtzis said.
"So our hope is that there will be full cooperation by all to make sure that it happens as smoothly as possible, without resulting in displacement, and ensuring the protection of civilians, ensuring that the basic principles of humanity will be respected on the ground."
The United Nations is in contact with all sides to explain where clinics, schools, water points, markets and residential areas are located, and to urge them "to stay away from civilian people", Moumtzis said.