Syrian President Bashar al-Assad tops a list of 20 sample war crimes indictments of government officials and rebels drafted by experts for prosecution someday, a former international war crimes prosecutor said on Tuesday, according to Reuters.

The list has been handed to the International Criminal Court, citing for each incident a specific violation of the Rome statute under which a suspect could be charged, according to David Crane, an ex-chief prosecutor of the Special Court for Sierra Leone and now head of the Syria Accountability Project, Reuters reported.

A separate team of U.N. investigators has drawn up four confidential lists of war crimes suspects on all sides in Syria, but declined to reveal any names, according to Reuters.

Crane said the list compiled by his expert group included members of Syria's military and political elite plus Islamist rebel groups ISIS and al-Nusra Front, although he gave no names beyond Assad, Reuters reported.

"We have about 20 indictments of those who bear the greatest responsibility. This is a neutral effort. We're not just going after Assad and his henchman, we are actually documenting all incidents on both sides," Crane told Reuters.

Images taken by a Syrian military police photographer codenamed Caesar, published in January, supplied "clear evidence" showing the systematic torture and killing of about 11,000 detainees in conditions that evoked Nazi death camps, former prosecutors including Crane have said, according to Reuters.

"We rarely get this type of evidence, most of it is circumstantial," Crane said of the 55,000 photographs of bodies, many with gouged-out eyes and bearing signs of starvation, Reuters reported.

"Make no mistake about it, these photographs could not be faked. This takes responsibility for what happened up the ladder of responsibility. It is not an act of a maverick colonel or a mad major, this is government policy," said Sir Desmond de Silva, co-author of an analysis of the "Caesar" photos and another former Sierra Leone chief prosecutor, told the panel, according to Reuters.