Fighters belonging to the Islamic State of Iraq and Syria are expected to be added to a list being drawn by the United Nations to document possible war crimes indictees, the chief rights investigator said, Al Arabiya reported.
Brazilian chief investigator Paulo Pinheiro, who heads the U.N. Commission of Inquiry on Syria, said incriminating evidence against ISIS would be easier to collect because "there are indications of a strong chain of command."
"They are good candidates for the list," said Pinheiro, who briefed the U.N. Security Council on the results of their inquiry.
"I can assure you that we are collecting information on perpetrators from all sides including non-state armed groups and ISIS," he told reporters.
"I am not in a position to say who is winning the World Cup of human rights violations. Both sides are doing horrific things and they will continue if there is no accountability."
After a successful offensive in Iraq, ISIS fighters last month declared an "Islamic caliphate" in territory it controls in Iraq and Syria and vowed to expand.
They have been accused by rights groups of summary executions, abductions, murder among other atrocities.
Pinheiro said there were lots of Syrian rebels are defecting to join the ultra-hardline Islamic insurgents.
ISIS was undergoing a "Syrian-ization," he said in statements carried by Reuters.
"What began with a lot of foreign fighters, now you have authentic Syrian citizens," Pinheiro told reporters. "We are seeing more Syrians coming to ISIS than before."
He said there was an estimated 10,000 to 15,000 foreign fighters with various groups battling Syrian President Bashar al-Assad's troops.
The U.N. Security Council failed in May to agree on a formal request to the International Criminal Court to take action toward prosecutions after Russia and China vetoed the move.
Set up in September 2011, the Commission of Inquiry has collected witness accounts, satellite photographs and other documents to build up its caseload of human rights violations, although none of the members have traveled to Syria.
The war crimes list being drawn up by the inquiry remains confidential but Pinheiro said it included intelligence chiefs, heads of detention centers that have torture chambers and military commanders "who target civilians."