ISTANBUL—United Nations officials went ahead with an ill-fated aid delivery to a rebel-held town in northern Syria Monday despite warnings by their own staff and Western-backed rebels that the area wasn’t safe, according to communications seen by The Wall Street Journal.

Airstrikes Monday night on the town of Urem al-Kubra just outside the city of Aleppo destroyed 18 of the 31 trucks in the convoy and killed about 20 people accompanying them. The U.S. on Tuesday blamed the attack on Russia, the Syrian regime’s key international...