By Andrew North
BBC correspondent in Herat
The head of the United Nations drugs control agency has told the BBC that efforts to tackle Afghanistan's growing drugs trade are failing.
Poverty among farmers is one of the causes of drugs production
Antonio Maria Costa said that the country would face a dangerous future if it was not brought under control.
According to Mr Costa, up to 90% of the opium from Afghanistan's poppy fields is turned into heroin inside the country's borders.
Most of the heroin consumed in Britain originates from Afghanistan.
'More serious problem'
The executive director of the UN Office on Drugs and Crime has warned about the threat from the Afghan drugs trade before.
But on his latest visit to the country Antonio Maria Costa said the problem was getting ever more serious, despite international efforts - led by Britain - to tackle it.
The UN says a reappraisal of the drugs problem is required
Some drug profits may be going to terrorists, and although it is officially illegal, opium production is growing more widely than ever.
Poverty among farmers is often seen as one of the causes.
Mr Costa accepts it plays a role, but he says there are more important factors.
"The problem is becoming ever more serious," he said.
"Cultivation is spreading and eradication measures undertaken by the government are not succeeding.
"Corruption is widespread and it is a major lubricant for trafficking and illegal activities such as money-laundering."
But many of the police commanders Mr Costa met over the past few days said they did not have the resources or the manpower to fight the drugs traffickers.
That is why Mr Costa is calling for a reappraisal of the way the Afghan authorities and their international backers deal with the problem.