"We don't know what is going to happen. We are preparing for the worst," the UN regional humanitarian coordinator for Syria, Panos Moumtzis, said in Geneva.
Turkey has threatened an offensive in Syria against Kurdish forces which it considers terrorists.
Moumtzis said there were "a lot of unanswered questions" about the consequences of the operation.
He added that the UN was "in contact with all sides" on the ground.
But he made clear his office did not have advance warning about the US decision, which effectively abandons the Kurds, Washington's main allies in the long battle against the so-called Islamic State group.
Moumtzis said the UN's priorities were to ensure that any prospective Turkish offensive not result in new displacements, that humanitarian access remain unhindered and that no restrictions be put in place on freedom of movement.
He said the UN was asking "that planners, the military, take into account that fact that... there (are) hundreds of thousands of people who live in this area."
The UN currently provides emergency relief to about 700,000 people in northeast Syria.
Moumtzis said the UN was "worried" about possible disruptions to humanitarian aid routes and had made contingency plans to address additional civilian suffering.
He added that Turkey had a good record of facilitating aid
deliveries throughout the Syrian conflict and that the "de facto authorities" in the northeast, meaning the Kurds, had also been cooperative with UN humanitarian operations.
Turkey has sent reinforcements to the border in recent weeks, and President Recep Tayyip Erdogan said Monday the long-threatened offensive could "come any night without warning".
His comments came after Foreign Minister Mevlut Cavusoglu said on Twitter that Turkey was "determined to ensure our country's existence and security by clearing terrorists from this region".
He was referring to the SDF, or Syrian Democratic Forces, which have ties to Kurdish militants inside Turkey and which Ankara considers a terrorist organisation.