The Russian military and a main Syrian opposition group traded allegations Friday about whether new evidence indicates that either rebels or Syrian military used chemical weapons in the northern area of Aleppo where government forces are trying to regain control of areas they recently lost to insurgents.

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov said that ministry experts have found unexploded ordnance and fragments of munitions containing chlorine and white phosphorus on Aleppo’s southwestern outskirts. Konashenkov said the discovery proves the militants have used chemical weapons against civilians and Syrian army soldiers.