A new body is being set up at the United Nations in Geneva to prepare prosecutions of war crimes committed in Syria, U.N. officials and diplomats said on Thursday.

The General Assembly voted to establish the mechanism in December and U.N. Secretary-General Antonio Guterres is due to name a judge or prosecutor as its head this month.

"We expect to start very, very shortly with just a handful of people," a U.N. human rights official told Reuters.

The team will "analyse information, organise and prepare files on the worst abuses that amount to international crimes - primarily war crimes, crimes against humanity and genocide - and identify those responsible", she said.

While it would not be able to prosecute itself, the idea is to prepare prepare files for future prosecution that states or the International Criminal Court in The Hague could use.

The focus on prosecutions means evidence collected since 2011 by a U.N. Commission of Inquiry may be sharpened into legal action.

The COI has issued 20 reports accusing the Assad government, rebel forces and Islamic State of mass killings, rapes, disappearances and recruiting child soldiers.

It too lacks a prosecutorial mandate, but has denounced a state policy amounting to "extermination", and has compiled a confidential list of suspects on all sides, kept in a safe.

Rights watchdog Amnesty International said last week the Syrian government executed up to 13,000 prisoners in mass hangings and carried out systematic torture at a military jail. Syria denied the report, calling "devoid of truth".

A Swedish court on Thursday sentenced a former Syrian opposition fighter who now lives in Sweden to life in prison for war crimes.

A U.N. report in January put the start-up budget for the new team at $4-6 million. So far $1.8 million has been donated, the U.N. official said. Funding is voluntary, posing a major challenge.

BUILDING CASES

The United Nations aims to recruit 40-60 experts in investigations, prosecutions, the military, and forensics, diplomats said.

"It's a very important step. It will not only allow court cases but also help us preserve evidence if there are cases in the future," a senior Western diplomat said.

Legal experts and activists welcomed the initiative.

"The focus is on collecting evidence and building criminal cases before the trail goes cold," said Andrew Clapham, professor of international law at Geneva's Graduate Institute.

Jeremie Smith of the Cairo Institute of Human Rights Studies said the United Nations must lay the groundwork for prosecutions ahead of any "exodus" of perpetrators when the war ends.

"This is the only way to make sure criminals don't get away by fleeing the scene of the crime."

The new team will seek to establish command responsibility.

"This is mass collection of information on all sides with a view to prosecution in the future by the ICC (International Criminal Court), national courts or in some completely new international tribunal that would be created," Clapham said.

Many national courts could pursue suspects using its dossiers, he said. States that have joined the ICC could bring cases to the Hague court, without referral by the Security Council.