Liberia's militias have refused to disarm their fighters unless they are given more top posts, says the United Nations special envoy.
Armed children, often high on drugs, still roam across large parts of Liberia
Jacques Klein said that two rebel groups and the former government walked out of discussions on disarming some 40,000 fighters, many of them children.
The disarmament programme is due to start next week after 14 years of war.
Former BBC West Africa correspondent, Mark Doyle, says this is the most serious problem the UN has encountered in Liberia since it established a peacekeeping force there earlier this year.
UN peacekeepers have brought relative peace to the capital, Monrovia, but the rest of the country remains volatile.
"I am appalled that a small number like this can obstruct what is good for 3.5m people," Mr Klein said.
He warned that the international community would take action against those who violated the peace accords signed in August, in neighbouring Ghana.
The walk-out has also been condemned by the United States embassy in Monrovia.
Well informed sources in Monrovia, who requested anonymity, said senior officials in the factions appeared to be scared of fighters in their ranks who were demanding jobs and money before giving up their weapons.
But Emmanuel Lomax, from the former government of Charles Taylor, said that Liberia's interim leader Gyude Bryant was to blame for the problems with the peace process.
Peacekeepers have brought relative peace to Monrovia
He said that Mr Bryant, agreed to as chairman of a power-sharing government by all sides, was not appointing those nominated by the three groups to key positions.
He said that under the Accra peace accords, the three groups named assistant ministers and deputy managing directors of public corporations.
He also warned the tough-talking Mr Klein against making threats.
"It's not intimidation that will serve as inducements for fighters to disarm," he said.