Ukrainian officials said Tuesday that they’re investigating unconfirmed reports that the Russian military has used “phosphorus munitions” in the key southern port city of Mariupol, which would be the first time chemical weapons have been used in the seven-week war.

Chemical weapons were used in Syria’s ongoing civil war, where Russia assisted the government’s army against opposition forces and civilians. Ukrainian officials said they’re concerned that Russian President Vladimir Putin’s forces are resorting to the same in Ukraine.

Russian troops have been attacking Mariupol for weeks due to its strategic location and tactical importance related to the rest of the battle-scarred nation. Moscow began the fighting on Feb. 24.