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* U.N. experts draw up fourth Syria suspects list
* Suspects at "higher levels" of government
* Rebels also report alleged crimes by other rebel groups
* Torture photographs not verified but U.N. investigating
By Stephanie Nebehay
GENEVA, Jan 24 (Reuters) - U.N. war crimes experts have documented more torture and killings by both sides in Syria and are confident they can build a case that could be taken up by the International Criminal Court, a leading member of the team said on Friday.
They are drawing up a fourth confidential list of suspects, either individuals or units linked to crimes committed since July, Karen Koning AbuZayd, an American expert serving on an independent commission of inquiry set up by the United Nations in 2011, said in an interview.
U.N. human rights chief Navi Pillay said in December that evidence collected by the investigators implicates President Bashar al-Assad, later denying that she had direct knowledge of their secret lists.
AbuZayd said the lists went up to "higher levels" of the Syrian government, declining to be more specific in the interview in Geneva, where the first talks involving the warring parties are expected over the coming week.
Foreign fighters in Syria, mainly Islamist groups, have their "own agenda", sometimes setting up sharia courts that issue summary sentences carried out immediately, including executions, AbuZayd said.
"Civil wars can be pretty bad, but people coming in from outside with radical agendas really don't give a damn what they do to things or people in that wonderful country that Syria was."
Photographs allegedly taken by a Syrian military police photographer said to show the systematic torture and killing of about 11,000 detainees are not deemed admissible evidence for now, although the team is trying to find out more, AbuZayd said.
"We've told those people who have this information, to whom it was given, that whatever they would want to share with us, we would be following up.
"They claim to have numbers and names and so on and that families have identified some of these people. But they have to be very careful because the families are still inside (Syria)."
The 55,000 images provided by the photographer, who fled Syria after passing the pictures to Assad's opponents, show emaciated and mutilated corpses.
"As far as we understand, those things are done, as described. But where these things came from and who the person was and all of that, we just don't know," AbuZayd said.
"For us of course it is also a single source which we wouldn't use because it is only a single source," she said at the Geneva-based U.N. human rights office.
The U.N. commission of inquiry has previously documented a number of cases of torture that led to death, similar to those described in Britain's Guardian newspaper on Monday. Reported deaths in custody rose markedly during 2013, it has said.
According to the U.N. findings, the Syrian government and its intelligence agencies have used widespread, systematic torture to interrogate, intimidate and punish people seen as opponents. Torture has been used in detention centres, security branches, prisons and hospitals.
Documented methods used by the government include electric shocks, severe beating while in stress positions, cigarette burns, mock executions, sleep deprivation, and psychological torture such as threats to rape family members, it says.
REFUGEES BEARING SCARS
AbuZayd, who has interviewed Syrian refugees bearing scars on their backs and gauged eyes from mistreatment in detention, said: "It is certainly not the first time that these things have been identified.
"We've had many more interviews over the years, the two and a half years that we have been collecting evidence of these kind of things happening in detention centres."
The U.N. investigators have also documented torture and killings by rebel forces and said in September that hardline rebels and foreign fighters invoking jihad, or holy war, have stepped up killings, executions and abuses in the north.
"Since there is tension between and among the various opposition groups, we're getting more information about opposition groups from other opposition groups. So there is more information on both sides now," AbuZayd said.
Paulo Pinheiro, who leads the inquiry, is among four commissioners who include AbuZayd, former U.N. war crimes prosecutor Carla del Ponte and Vitit Muntarbhorn. They are finalising their next report, due to be issued on February 20.
Their team of more than 20 investigators has interviewed 500 refugees, defectors and people still in Syria since July, bringing the total number of testimonies gathered to 2,600 since the inquiry began its work in Sept 2011.
The International Criminal Court - the world's first permanent war crimes court - has so far been powerless to act on Syria because Damascus did not sign up to it and the U.N. Security Council has been deadlocked as Russia and China oppose referring Syria to the Hague-based court.
"It has to come through the Security Council in the case of Syria. That's been the problem all along. How many times have we challenged the Security Council about this? They are the ones that have to take action," AbuZayd said.
"We have a lot of investigators that have been through the ICC, they've worked there," she said. "They know the rules, what kind of evidence is needed. I think we have really solid stuff that will come out from them."
The U.N. has an "enormous database" that would be available to any judicial body deemed objective and appropriate, she said. The whole point of the team's mandate was accountability.
"Otherwise what we're doing doesn't make any sense, it has no meaning unless someone is called to account one day for all this information that we are collecting and all these abuses we are documenting," she said.
Both the Syrian government and opposition have accused each other of crimes, demanding perpetrators be held to account.
"They both want accountability about what the others do, rather than themselves," she said.
(Reporting by Stephanie Nebehay; editing by Philippa Fletcher)