Brazilian expert Paulo Pinheiro told reporters in Paris that it was hard to distinguish between the good and the bad so-called “opposition” in Syria.
"It was said the opposition forces were angels, but there is only a minority of fighters with a democratic history who believe in the Syrian mosaic and want a state for all," he said.
"The majority of opposition forces are very far from having democratic thoughts and have other aspirations,” Pinheiro added.
"There is a very complicated distinction between the bad and the good opposition forces," he said.
Pinheiro leads an independent team of some two dozen experts mandated by the United Nations that documents crimes committed during the conflict.
Last week, the European Union (EU) effectively lifted an arms embargo that could allow western countries to arm foreign backed terror groups in Syria.
Pinheiro declined to comment on that decision, but suggested identifying groups “acceptable” to the West was difficult.