Secretary-General Ban Ki-moon is expressing alarm at "the weakening taboo" against using chemical weapons in Syria where an international watchdog agency says it is studying several recent cases of alleged use of the banned agents.

The U.N. chief said in a letter to the Security Council transmitting a report from the Organization for the Prohibition of Chemical Weapons that he is gravely concerned that the use of chemical weapons could become "normalized in this or any conflict, present or future."

Ban said in the letter, circulated Monday, "It is imperative that those responsible for the use of chemical weapons should be held accountable."

The OPCW said its fact-finding mission is studying four widely reported allegations of chemical weapons use. It is already investigating the alleged use of chemical weapons in Aleppo on Aug. 2, responding to a request from the Syrian government.