Between 2001-03, Hammarberg acted as Regional Adviser for Europe, Central Asia and the Caucasus for the UN High Commissioner for Human Rights. For several years, he was the Swedish Prime Minister's Personal Representative for the UN Special Session on Children, as well as the Convener of the Aspen Institute Roundtables on "Human Rights in Peace Missions". Between 1996 and 2000, he was the appointed representative of the UN Secretary General, Kofi Annan, for human rights in Cambodia. He also participated in the work of the Refugee Working Group of the multilateral Middle East Peace Process.