Coca production in Colombia grew by 8% in 2005 - the first rise in five years, the UN Office of Drugs and Crime says.
The US has backed an aggressive anti-coca drive
The production of coca, the raw material for cocaine, reached 86,000 hectares last year, UNODC found.
Despite the rise, it is half of Colombia's cultivation in 2000, which then stood at 163,000 hectares.
Colombia needs more international help to find alternative livelihoods for ex-coca farmers, one of the report's authors, Anja Korenblik, said.
Since 2000, the United States has spent more than $4bn on the fight against drug-trafficking in Colombia.
This has involved the aerial spraying of herbicides to kill the crops, as well as by law enforcement.
But despite such eradication efforts, the area under coca cultivation in Colombia rose by 6,000 hectares between 2004 and 2005, UNODC found.
Beating the cartels
"In Colombia in 2005, we found 86,000 hectares of coca bush, that is an 8% increase on 2004 when we saw 80,000 hectares," Mrs Korenblik told the BBC's Americas Service.
"That's almost half of the cultivation in the year 2000 when it was more than 160,000 hectares so it's really an impressive decline."
But she said the research showed more economic aid was required for poor farmers who grow coca on behalf of the drug cartels.
"What is needed is for the international community to support Colombia in identifying alternative livelihoods for ex-coca farmers," she said.
"This is not happening enough and that's probably why it's difficult for all the Andean countries to sustain their reductions in coca cultivation that they have seen," she added.
Colombia remains the world's largest producer of cocaine, although its share has dropped to 54% from 74% in 2000.
Coca production in Peru and Bolivia fell by 8% and 4% respectively in 2005, UNODC found.