GENEVA (Reuters) - Allegations of a chemical weapons attack in Syria are "exceptionally grave" and must be investigated as soon as humanly possible, U.N. human rights chief Navi Pillay said on Thursday.

"The use of chemical weapons is prohibited under customary international law," she said in a statement. "This absolute prohibition applies in all circumstances... it is binding on the Government despite it not being party to the 1993 Chemical Weapons Convention. It is also binding on anti-Government armed groups."

(Reporting by Tom Miles; Editing by Sonya Hepinstall)