Outgoing Syria war crimes investigator Carla Del Ponte said on Monday she had quit her post out of frustration over "total impunity", in a fiery farewell speech.

Del Ponte, an accomplished war crimes prosecutor, announced last month that she was leaving the Commission of Inquiry (COI) on Syria, a UN-backed panel that has collected evidence of alleged atrocities committed in the country since the outbreak of civil war in 2011.

"I resign to put an end to my frustration," Del Ponte told the Human Rights Council, after the COI presented its latest report.

"Seven years of crimes in Syria and total impunity. That is not acceptable".

Del Ponte, a Swiss national, worked on the international tribunals that prosecuted war crimes, crimes against humanity and genocide in the former Yugoslavia and Rwanda.

She said that when she joined the COI she did not anticipate that the international community would fail to set up a court capable of trying crimes committed in Syria.

The commission has repeatedly urged the United Nations Security Council to refer Syria to the International Criminal Court, an effort that has been blocked by the Damascus regime's ally Russia.

"Why is it not possible to have a tribunal?" Del Ponte asked, addressing some of her farewell remarks directly at Syria's UN ambassador Hussam Edin Aala.

The COI has accused all sides -- including the government and rebels -- of committing war crimes in Syria.

Its most recent report, submitted to the rights council on Monday, charged Damascus with carrying out a chemical attack which killed dozens of people in Khan Sheikhun in April.

A new UN body -- called the International, Impartial and Independent Mechanism -- has recently started work in Geneva with a mandate to prepare cases against specific individuals.

Del Ponte called that effort "a little step towards justice".

But the mechanism's chief, Catherine Marchi-Uhel of France, cautioned earlier this month that her office was not a court and had no power to bring international charges.