"As we look around the world at the end of 2013, we see examples of situations where that readiness of the international community to act in time is already being sorely tested," Pillay told a news conference that touched on trouble spots around the world.

The statement by Navi Pillay, who heads the U.N. Office of the High Commissioner for Human Rights, adds to the pressure on Syria for action ahead of a key peace conference planned for Geneva in January . The conference, brought on by combined U.N., U.S. and Russian diplomacy, would for the first time bring the Assad government and opponents together for face-to-face negotiations .

A growing body of evidence collected by U.N. investigators points to the involvement of senior Syrian officials, including President Bashar al-Assad, in crimes against humanity and war crimes, the United Nations' top human rights official said Monday.

Pillay said the Syrian abuses — suspected massacres, chemical attacks, torture, rape and a litany of other horrors — are being well documented by a panel of expert U.N. investigators.

"They've produced massive evidence," she said. "They point to the fact that the evidence indicates responsibility at the highest level of government, including the head of state."

Syrian Deputy Foreign Minister Faisal Mekdad dismissed Pillay's remarks.

"She has been talking nonsense for a long time and we don't listen to her," he told The Associated Press in The Hague.

Pillay said the lists of suspected criminals are handed to her on a confidential basis and will remain sealed until requested by international or national authorities for a "credible investigation," and then possibly used for prosecution.

She said she worries about striking the right balance in determining how long to keep the information secret. The evidence "rightly belongs to the people who suffered violations," she said, adding that the lists also must be kept sealed "to preserve the presumption of innocence" until proper judicial investigation which may lead to trial.

Any prosecution before the International Criminal Court at The Hague appears to be a long way off because Syria is not a member of the court. This means the U.N. Security Council itself would need to refer the matter to the court. The 15-nation council did so once before in the case of Sudan, in 2005, but would be unlikely to do so in Syria's case because its key ally Russia is one of the council's five permanent members that wield a veto.

Pillay and the four-member U.N. panel on Syria war crimes, chaired by Brazilian diplomat and scholar Paulo Sergio Pinheiro, have previously said Assad's government and supporters, along with the rebels who oppose them, have committed war crimes during the nearly 3-year-old civil war in Syria.

But this time, Pillay specifically referred to the president — though she was careful to say she had not singled him out as a possible suspect on the secret lists.

Pillay said Syria and North Korea — the two countries being investigated by a U.N. panel — represent two of the world's worst human rights violators, but she also cited concerns with Central African Republic, Bangladesh and other regions.