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.

A team of investigators sent by U.N. Secretary-General Ban Ki-moon must be given access "without any delay or obfuscation," she said in a statement.

"The use of chemical weapons is prohibited under customary international law," she said. "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."

The prohibition on chemical weapons is one of the strongest and most unequivocal in international law, along with genocide and torture.

If the allegations are proven to be true they would top the list of charges of war crimes and crimes against humanity already levelled at both sides in Syria, potentially defining the conflict as the Srebrenica massacre did in the Bosnian war.

"These allegations are exceptionally grave and need to be comprehensively proved or disproved as soon as is humanly possible," Pillay said.

Pillay's staff in the region heard from well-placed sources that in addition to hundreds of reported civilian deaths, there were thousands of injured in need of medical care and humanitarian aid, the statement from her office said.

(Reporting by Tom Miles; Editing by Sonya Hepinstall)