By Stephanie Nebehay
GENEVA (Reuters) - Suspected war criminals in Syria will face justice for gross violations against civilians someday as the evidence against all sides grows, the U.N. human rights chief said on Thursday.
Zeid Ra'ad al-Hussein, speaking on the eve of the fourth anniversary of the start of the conflict, denounced the "bloodlust" of Islamic State forces controlling large parts of Syria and Iraq who have beheaded Western hostages and burned alive a Jordanian pilot.
"We have a massive amount of evidence, there are lists with people's names on them," Zeid told a Geneva audience in a joint interview with U.N. refugee boss Antonio Guterres by CNN's Christiane Amanpour.
"Justice will be done, it may not be immediate, but hopefully as we go through the transition from total impunity to justice being realized, we will honor the victims and the kin of the victims," he said at the annual event commemorating the late U.N. Iraq envoy Sergio Vieira de Mello.
A U.N. commission of inquiry led by Paulo Pinheiro has gathered testimony and evidence, compiling five secret lists of suspects whom it says include Syrian government military and security officials and rebel commanders.
The investigators said last month they plan to publish names of suspects and push for new ways to bring them to justice.
"There will be less and less impunity. I am absolutely sure evidence will find its way to a prosecutor who will be able to prosecute at least the worst crimes," Zeid said.
The U.N. Security Council remains "essentially paralyzed" and unable to take steps to end the war that has killed more than 200,000, Guterres said.
Pinheiro and his team, due to present their latest report to the Human Rights Council on Tuesday, have suggested setting up an international ad hoc tribunal as major powers have been unwilling to refer crimes to the International Criminal Court.
Pinheiro, speaking at a separate event, decried the Assad government's dropping of barrel bombs on residential areas and shelling by ISIS and al-Nusra of populated areas - potential war crimes.
"The last time the Security Council decided to say something on accountability was one year ago. The international community must be creative. It is time. Victims cannot wait," he said.
Andrew Clapham, an international law professor, said a new international ad hoc tribunal was a good alternative to the ICC.
"It would send a message to those currently conducting and planning these operations and foreign fighters joining ISIS that eventually they could be prosecuted for war crimes and could be sent to prison for a long time."
(Editing by Hugh Lawson)