The United Nations says it has received reports of three humanitarian workers killed by an airstrike on the outskirts of Aleppo in Syria.

The deputy spokesman for Secretary-General Ban Ki-moon told reporters that officials do not think the three workers killed Wednesday were U.N. personnel.

Farhan Haq did not say who launched the airstrike.

He says the U.N.'s humanitarian agency is "extremely concerned about the intensification of fighting and its impact on the civilian population" after reports of more airstrikes in Aleppo governorate in recent days.