"Just because we have had one veto in the (UN) Security Council should not stop any of us from moving forward and seeking justice for the people of Syria," said David Crane, the former international prosecutor who indicted Liberian president Charles Taylor.
China and Russia last month vetoed a draft UN Security Council resolution to refer Syria to the ICC for crimes committed by both sides in the three-year civil war.
But Crane insisted there were other options for seeking justice, including through special national or regional courts.
"All options are on the table," he insisted to an event on the sidelines of a UN Human Rights Council session in Geneva.
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"I know it's a natural desire to seek quick justice. But justice will be done, over time," he said.
Desmond Lorenz de Silva, another former chief prosecutor of the Special Court for Sierra Leone, told AFP that a number of special courts -- including the famous Nuremberg trials against suspected Nazi war criminals after World War II -- were created outside the UN system.
Both Crane and de Silva were among the legal experts behind the so-called "Caesar Report" released at the start of the year, containing some 55,000 photographs depicting the tortured and abused bodies of around 11,000 people it said had died in Syrian jails between 2011 and 2013.
They maintain the report and the horrifying pictures, depicting twisted, mangled and emaciated bodies provided clear proof of war crimes and crimes against humanity.
One victim confirmed their veracity.
"We didn't see pictures. We saw real people like this in front of us," said Ala Al Cizawi, adding she had been beaten, tortured with electricity and hung from her arms for 12 hours at a time in detention centres last year.