Carla del Ponte, a former UN chief prosecutor, has been appointed a member of a United Nations commission investigating war crimes in Syria.
The commission is gathering evidence for possible future trials of individuals and military units suspected of committing abuses in the 18-month-old conflict in which forces loyal to President Bashar al-Assad are seeking to quell an uprising against him.
Switzerland proposed that del Ponte, a former Swiss attorney-general, become a commissioner and lobbied hard on her behalf, overcoming resistance by some countries that found her too controversial, diplomats said.
Laura Dupuy Lasserre, president of the UN Human Rights Council, said she had appointed del Ponte and Vitit Muntarbhorn to the Commission of Inquiry to join Brazilian Paulo Pinheiro and American Karen Abuzayd.
"Both have a long track record which has been recognised at the highest possible level internationally," she told the Council at the end of a three-week session. "Their reputation is of impartial, independent and objective human rights work."
The appointment of such an experienced lawyer as del Ponte suggests the United Nations is determined to try to build a solid criminal case and bring human rights violators to justice.
Del Ponte's eight years at the Hague-based court International Criminal Tribunal for the former Yugoslavia were dominated by the pursuit and trial of former Yugoslav President Slobodan Milosevic, who died in 2006 before sentencing.
"She brings strong investigative skills and an ability to better systematise and use the huge amount of information that the Commission of Inquiry is putting together with a view to prosecution one day," European Union (EU) Ambassador Mariangela Zappia told Reuters.
Peter Splinter, a spokesman for Amnesty International, said: "She will bring valuable knowledge and experience to an already strong commission team."
Muntarbhorn is a veteran expert from Thailand who has served as UN special rapporteur on North Korea.
Earlier, the Council extended the mandate of the Syria inquiry by another six months and condemned violations by government forces and pro-Assad militias but also by rebel fighters.
The 47-member forum adopted a resolution by a vote of 41 states in favour, with three states - China, Cuba and Russia - against and three abstentions.
Russia, Syria's ally, voiced concerns about the probe.
"We regret the fact that it has been difficult to achieve official access to Syria, one has to cope with rumours and fabricated data," Russian diplomat Vladimir Zheglov said.
"Therefore, we call upon the commission to act objectively and in a non-politicised way and to use verified facts."
Faysal Khabbaz Hamoui, Syria's ambassador, rejected the text of the resolution submitted by seven Arab states and backed by Western powers as "highly-politicised and selective".
He accused Islamist "terrorists" of fuelling the violence in his country, where activists say 30,000 people have been killed.
The international inquiry led by Pinheiro, set up a year ago, has interviewed more than 1100 victims, refugees and defectors, documenting massacres including the one at Houla near Homs in May. But it has been denied access to Syria.
Earlier this month, it expanded its secret list of Syrians suspected of committing war crimes and crimes against humanity, including executions and torture, who it says should face future prosecution.