The United Nations Security Council has unanimously approved a force of up to 15,000 peacekeepers for Liberia.
The resolution, proposed by the United States, sets up a UN mission in Liberia to monitor last month's peace agreement between the government and rebels.
Liberia as a whole remains highly unstable
The main task of the force will be to restore security and to devise a plan to disarm more than 30,000 militiamen, including child soldiers, who are still active in the countryside.
In August, Liberia's two main rebel groups signed a power-sharing deal with the interim government to end four years of bitter civil war.
Nigeria has been the main player in peace efforts in Liberia, where its troops make up most of a 3,500-strong West African peacekeeping force - Ecomil.
Its peacekeepers have helped subdue violence in the capital, Monrovia, but not in rural areas.
More than 1,000 civilian police will be included in the UN peacekeeping mission.
No countries have yet committed troops to the proposed force, but the UN envoy to Liberia is currently holding discussions with several European governments.
The 15-member council created the mandate for a year, subject to renewal, but it is expected to be three to four months before the force is deployed.
"The general consensus is that this is a failed state," Jacques Paul Klein, the chief U.N. envoy for Liberia, said earlier in the week.
"Now we have to rebuild the state."
"The former troops are robbing, raping. This situation will get worse before it gets better as the fighting is over and there is not yet any UN mission in place," Klein added.
The BBC's Mark Doyle, until recently based in Liberia, says the news will be greeted with joy by most ordinary Liberians, who see the planned arrival of UN peacekeepers as the best chance they have had for a generation.
At least half a million Liberians are officially described as displaced, but in reality almost every one of the country's 2.7 million people have been adversely affected by the conflict - either through a death in the family or through the impoverishment that war brings.
The displaced have fled the countryside, where government and rebel militias rob them, to take refuge in squalid refugee camps.
But Liberia is a failed state from top to bottom, and everything needs rebuilding, our correspondent adds.
The force is expected to operate under a Chapter Seven mandate - a technical term that would give them the most robust mode of operation available under the UN charter.
In a report to the Security Council this week, UN Secretary General Kofi Annan said that, despite positive political developments in the Liberian capital, Monrovia, the country as a whole remained highly unstable.
Mr Annan says that, ultimately, the success or failure of the disarmament and demobilisation process rests in the hands of the warring parties themselves.