A UN panel of investigators have collected evidence showing the scale and viciousness of the human rights abuses being perpetrated by both sides in the Syrian conflict.

Navi Pillay, chief of the UN Office of the High Commissioner for Human Rights, on Monday said the scale and viciousness of the abuses almost defies belief. She said reports have been documented by a UN panel of expert investigators.

"They point to the fact that the evidence indicates responsibility at the highest level of government, including the head of state," Ms Pillay said.

This is the first time evidence has been collected by an UN-mandated commission that directly implicates President Assad and members of his inner circle in war crimes and crimes against humanity.

It is estimated that the 33 months of brutal conflict have killed an estimated 126,000 people, according to new figures released by the Syrian Observatory for Human Rights, a Britain-based monitoring group.

The four-member UN investigative team which has been probing human rights violations in Syria since fighting broke out has blamed both the regime and rebels for war crimes.

Thus, for the first time, they have prepared a confidential list of suspected perpetrators.

As the investigators did not have direct access to Syria, the team had to rely on more than 2000 interviews in neighbouring countries or by phone or Skype.

Ms Pillay warned that the effort to destroy Syria's massive chemical weapons stockpile should not distract international attention from the tens of thousands killed with conventional weapons.

The UN human rights chief told reporters in Geneva on Monday that the names of the perpetrators would remain sealed until they are handed over to a credible national or international investigation body.