By Susannah Price
BBC correspondent at the United Nations
The UN General Assembly and the Security Council have agreed to set up a peacebuilding commission to stop countries slipping back into violence.
Mr Annan (l) says the commission will help the transition to peace
The idea was originally agreed by world leaders at a UN summit in September.
UN Secretary General Kofi Annan says the commission is another important step towards renewing the organisation.
He said previous efforts to build peace with humanitarian assistance and long-term recovery in war-torn countries had often been fractured.
Mr Annan said the new peacebuilding commission would now bring together all those parties involved.
This is the second tangible outcome from the package of UN reforms agreed at the world leaders' summit in September, following the creation last week of an emergency fund.
"The peacebuilding commission will help countries make the transition from war to peace. It will advise on recovery, it will focus attention on reconstruction and institution building," Mr Annan said.
"It will improve co-ordination both within and beyond the UN system. Perhaps most important of all, it will liaise with the international community to keep us all engaged in the long-term recovery effort."
The commission will have an organisational committee made up of members of the main UN bodies, top donors and top troop contributing countries and regional groups.
It will advise the Security Council on planning peace-building activities and will also help individual countries on the verge of lapsing back into conflict.
Some diplomats in the general assembly, which adopted the resolution by consensus, expressed concern that the Security Council would have too much authority in the commission.
However, the president of the general assembly, Jan Eliasson, called it an historic step, stressing that the commission's goal would be to decrease the number of countries falling back into conflict.