GENEVA — After more than six years of atrocities in Syria that have been exhaustively documented by human rights investigators, a former French judge will take on the task of preparing evidence that may eventually lead to war crimes trials.

The judge, Catherine Marchi-Uhel, was appointed late Monday by the United Nations secretary general, António Guterres, to lead the legal team, being established in Geneva, that will collect and preserve evidence of crimes for use by courts or an international tribunal.

The legal team, the International Impartial and Independent Mechanism, was created by a General Assembly resolution in December despite fierce resistance from Russia, which had repeatedly used its veto as a permanent member of the Security Council to block criminal investigations of the conflict.

The selection of Ms. Marchi-Uhel surprised some human rights experts, who had thought citizens of Security Council permanent members and countries that are part of the international coalition fighting the Islamic State in Syria and Iraq would be excluded. They said that Russia, or other critics of Syrian war crimes inquiries, could question the impartiality of someone from a country, like France, that is both a member of the Council and involved in the conflict.