GENEVA – The top U.N. human rights official linked President Bashar Assad of Syria to war crimes and crimes against humanity for the first time Monday, citing evidence collected by her panel of investigators over the course of the 33-month-old conflict in that country.

The four-member panel investigating human rights offenses in Syria has produced "massive evidence" of the commission of war crimes and crimes against humanity, the official, Navi Pillay, the U.N. high commissioner for human rights, told reporters in Geneva. She went on: "They point to the fact that the evidence indicates responsibility at the highest level of government, including the head of state."

The panel, which has not been allowed to enter Syria, has gathered information from refugees and other sources. It has compiled lists of names of individuals, military units and intelligence agencies implicated in the human rights abuses committed on a wide scale since the conflict began in March 2011, with a view to ensuring that those responsible are brought to justice.

As long ago as February 2012, the panel found "reasonable grounds to believe that particular individuals, including commanding officers, and officials at the highest levels of government bear responsibility for war crimes and gross human rights violations."

The panel also found Syrian opposition groups implicated in war crimes and crimes against humanity, although on a lesser scale.

But panel members, led by Paulo Pinheiro, a Brazilian rights expert, have studiously avoided going further in identifying either the individuals or even the number of names on their lists.

Pillay later sought to clarify her comment, saying, "I have not said that a head of state is a suspect. I was quoting the fact-finding mission, which said that based on their facts, responsibility points at the highest level."

Her comments, however, drew a swift riposte from the Syria's deputy foreign minister, Faisal Mekdad. "She has been talking nonsense for a long time and we don't listen to her," he was quoted by the Associated Press as saying.

Pillay said the panel had handed her lists of names to be held securely at the human rights office in Geneva. Pillay also repeated demands that she and the panel have made that the situation in Syria should be referred to the International Criminal Court in the Hague.

"Accountability should be key priority of international community, and I want to make this point again and again as the Geneva 2 talks begin," she said, alluding to the second international conference on Syria scheduled to start in Geneva on Jan. 22.