Coca production in Colombia is more widespread than previously registered, the US government says.
The US has backed an aggressive anti-coca drive
A survey found efforts to kill the crop - the source for cocaine - by spraying fields with chemicals had succeeded.
But the US government, expanding the survey area, found an extra 26% of land under cultivation. It also found production was more dispersed.
Correspondents say this is a setback for the US. It has spent $4bn fighting Colombia's cocaine trade since 2000.
The BBC's Jeremy McDermott in Colombia says US policy appears to be failing, with cocaine yields improving, demand in the US remaining steady, and a more liberal approach to coca cultivation in Bolivia and Peru.
However the head of the US Office of National Drug Control Policy, John Walters, insisted it was working.
His office said the increase in land discovered to be under cultivation came about because the survey area had been increased, in an attempt to improve its comprehensiveness.
- In 2004, 114,100 hectares (440 sq miles) were found growing coca
- In 2005, in the same area, cultivation had fallen to 105,400 hectares
- However, the survey area was expanded in 2005 by 81%. In this new area, a further 39,000 hectares were found growing coca, making 144,100 hectares in total, an increase of 26% overall
"Where there was no spraying cultivation was up, where spraying is occurring, cultivation is shrinking," Mr Walters said.
In some areas, farmers had moved production to more remote and difficult plots, the report said.
Critics of US policy said it was just forcing poor farmers to look for new areas to grow their produce.
"Coca production was not going to be reduced just because fumigation flights spray some fields, as long as these farmers don't have any other economic options, except to cut down forests to grow coca somewhere else," said Adam Isacson of the Washington-based Center for International Policy.
Colombia is the world's biggest producer of cocaine, and has attracted billions of dollars of US funds as Washington tries to cut the supply of the drug to American users.
However the US has found that production is also increasing in Peru and Bolivia.
Mr Walters said the joint US-Colombian coca eradication plan would continue.
He said it was also helping stabilise Colombia, where a civil war, funded by cocaine profits, has raged for decades.
"The terrorist groups are weaker... they are receiving less money, murders are down, kidnappings are down," he said.