He came to wide international prominence as a key figure in the negotiations that led to the Oslo Accords, when he served as the Director of the Fafo institute. In 1993, he was appointed Ambassador and Special Adviser for the Middle East Peace process to the Norwegian Foreign Minister, and the following year, he became the United Nations Special Coordinator in the Occupied Territories at the rank of Under-Secretary-General. He briefly served as the Deputy Prime Minister and Minister for Planning and Cooperation of Norway in the Jagland cabinet in 1996, and returned to the United Nations, where he again became an Under-Secretary-General, serving as the UN Special Coordinator for the Middle East Peace Process and Personal Representative of the Secretary-General to the Palestine Liberation Organization and the Palestinian Authority from 1999 to 2004.