(Reuters)- 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.

The list has been handed to the International Criminal Court (ICC), 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.

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.

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.

"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.

He was speaking after taking part in a panel discussion about torture and other crimes committed in detention centers during Syria's civil war, which began with peaceful anti-Assad demonstrations in March 2011.

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.

"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.

"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.

A six-week offensive by ISIS against rival Islamists in eastern Syria has killed 600 fighters and driven 130,000 people from their homes, a London-based monitoring group said on Tuesday.

ACCOUNTABILITY

Crane, an American professor at Syracuse University College of Law in New York, launched its Syria Accountability Project in 2011 to document war crimes and crimes against humanity committed by all sides in Syria's conflict.

It now has 1,400 pages of credible allegations, with the date, place and unit suspected of committing crimes, he said.

"At the beginning, 90 percent of the violations were Assad; it's now 50-50 (percent)," Crane said, referring to crimes committed by rebel forces fighting to topple the Syrian leader.

Last month, Russia and China vetoed a U.N. Security Council resolution to refer the situation in Syria to the International Criminal Court for possible prosecution of war crimes and crimes against humanity.

De Silva said that if the Council remained blocked on the matter, a tribunal on Syria could be set up if more than two countries entered into a treaty. "It may in fact be the only realistic course that can be taken. What we can't have is international justice stalled in its tracks."

(Editing by Mark Heinrich)