Nigerian-led peacekeepers have ended fighting in Monrovia
Killings, rape and looting will continue unless peacekeepers are deployed in large numbers, says the United Nations representative to Liberia.
Jacques Klein said that 15,000 troops were needed to make last month's ceasefire agreement work.
"Unless we, this time, do this thing correctly, it potentially destabilises West Africa," he told a news conference in the capital, Monrovia.
The BBC's correspondent in Monrovia, Mark Doyle, says it seems likely that this will be approved by the UN, making it the world's largest peacekeeping operation.
The UN envoy described Liberia as the key to the stability of the whole of West Africa.
"I think there's a clear understanding by the region that this, this time, has to work," he said.
He also urged the United States to stay beyond its planned withdrawal of forces at the beginning of October.
Mr Klein has already named countries that may contribute troops, including Nigeria, Ireland, South Africa and several Asian states.
The West African peacekeeping force in Liberia now has 3,050 soldiers and is expected to reach its full force of 3,500 African troops by Wednesday.
The BBC correspondent says the scale of the task facing any force is huge, with civil society needing rebuilding and tens of thousands of fighters having lived by the gun for a generation.
Earlier, West African peacekeepers urged thousands of Liberians to return to refugee camps around the town of Totota, some 80km north of the capital, Monrovia, saying the area was safe.
A team from the Ecomil peace force went to the area on Thursday to investigate reports of fighting between rebels and government forces but found no evidence of the clashes, said the Ecomil chief of staff, Theophilus Tawiah.
Colonel Tawiah told the BBC's Network Africa programme that newly arrived peacekeepers from Guinea Bissau would be sent to the central town of Kakata by the weekend.
Some 2,000 troops have ended fighting around Monrovia but they have not yet deployed elsewhere in the strife-torn country.
Fighting has also been reported in the eastern Nimba county, stronghold of former President Charles Taylor, but Colonel Tawiah said Ecomil did not have a mandate to deploy there.
An interim government meant to lead Liberia to elections in 2005 is due to take over from the caretaker President Moses Blah in October.