United Nations Secretary-General Antonio Guterres appointed a French legal expert and former judge on Monday to head the UN investigative body that will help document and prosecute the most serious violations of international law in Syria, including possible war crimes and crimes against humanity.

UN spokesman Stephane Dujarric announced the appointment of Catherine Marchi-Uhel who has been serving as the ombudsperson for the Security Council committee monitoring sanctions against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group and al-Qaeda.

She was previously a judge in France and an international judge with the UN mission in Kosovo and at the Cambodia court prosecuting leaders of the Khmer Rouge. She also served as senior legal officer at the International Tribunal for the former Yugoslavia and worked in legal positions at the French foreign ministry.

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The 193-member General Assembly voted last December over strenuous objections from Syria and close ally Russia to establish a new body "to closely coordinate" with the Independent International Commission of Inquiry on Syria, which was established by the Geneva-based UN Human Rights Council. The commission said last year that war crimes are "rampant" in Syria.

The "International, impartial and independent mechanism," established under UN auspices, is mandated "to collect, consolidate, preserve and analyse evidence of violations of international humanitarian law and human rights violations and abuses and prepare files in order to facilitate and expedite fair and independent criminal proceedings" in the future.

The General Assembly asked the secretary-general to arrange for the speedy establishment of the independent body, but it took Guterres, who took office on January 1, six months to announce an appointment.

The investigative body will initially be funded by voluntary contributions. The assembly has urged all UN member states, especially parties to the conflict, to cooperate with it.

More than 320,000 people have been killed in Syria where a UN Commission of Inquiry has documented cases of torture, summary killings and other atrocities by all sides in the conflict.