The US government says the cultivation of opium poppies in Afghanistan fell by nearly half in 2005.
Counter-narcotic officials called it a significant decrease which was due in part to strong efforts by the Afghan government.
They were speaking as the US state department released its annual report on narcotics production worldwide.
But the officials said they were concerned that poppy cultivation would increase in 2006.
Nearly all of the heroin flowing onto the streets of Europe and Russia is made from opium poppies grown in Afghanistan. And by the time Afghan opium has been exported, it is worth about $2.8bn (£1.6bn) a year.
'Lured back'
The US government said the amount of land in Afghanistan cultivated for poppies in 2005 decreased by 48%.
But because of good weather, the yield on that poppy crop was very high, so the amount of opium actually exported fell by only 10%, it said.
Still, American counter-narcotics officials said they were pleased and surprised at this fall in cultivation.
They said it was because of concerted efforts by the Afghan government and its international partners. Many Afghans had heeded the call of President Hamid Karzai to stop growing poppies and had switched to alternative crops.
But the picture for 2006 looks more mixed. The officials said early indicators suggested poppy cultivation was once again on the rise this year.
Anecdotal evidence from village elders and governors, as well as surveys of farmers, suggested some Afghans would be lured back to growing poppies by the easy cash they brought.
The officials also suggested that insurgents in Afghanistan may be encouraging poppy cultivation as a means of destabilising the central government.
The state department's annual report said the US was most threatened not by Afghan heroin but by cocaine imported from south and central America, and by methamphetamine - the cheap and dangerous stimulant that is wreaking havoc in some American communities.
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