“There are several examples where we are quite sure that shells with chemicals have been used in a very sporadic way,” a western diplomat was quoted by AFP as saying.

Another diplomat said that “quite convincing” evidence was sent to UN Secretary-General Ban Ki-moon to back the accusations made against forces loyal to Assad.

The Syrian government is only willing to allow the UN to investigate what it claims was a rebel chemical attack near Aleppo last month. The opposition has blamed Assad’s forces for that strike and also wants the UN team to look into other alleged chemical attacks by the government.