The Afghan government has said it is investigating reports that an unidentified aircraft sprayed opium poppies with herbicide.
The US has suspended its plans to spray opium poppies from the air
It comes amid continuing controversy over how to curb Afghanistan's booming drugs trade.
The governor of Helmand province in the south of Afghanistan told the BBC that poison had certainly been sprayed, but he did not know who was responsible.
US diplomats in Kabul have said they do not know anything about the incident.
The US government had previously announced that it had suspended plans to use aerial spraying to destroy opium poppies, following opposition from the Kabul government and aid agencies.
It is the second time since November that Kabul has launched an investigation into allegations of aerial spraying.
The last inquiry proved inconclusive.
Both Washington, which had previously earmarked cash for aerial spraying programmes, and the British government, which leads the international counter-narcotics effort in Afghanistan, denied responsibility.
Washington has since shelved proposals for aerial spraying.
The drugs industry makes up 60% of Afghanistan's economy
But the debate is still raging over what else should be done, with aid agencies arguing that if widespread eradication is carried out before providing alternative livelihoods for poor farmers, it could destabilise the country.
Peter Marsden, of the British Agencies Afghanistan Group, said: "The Afghan government would certainly prefer to take its time and see what is possible in terms of building up the economy.
"The major concern really is over statements by the US that it plans to engage in manual eradication this year, and also over statements by the British government that they feel the need to be seen to be doing something quickly.
"They're worried that if one waits for too long, the narco-mafia economy will become entrenched.
"There's a lot of unease in many quarters about the best way forward, in terms of achieving a balance between eradication and alternative livelihoods."
Afghanistan's President Hamid Karzai has declared a holy war against drugs but objects to aerial spraying as a health hazard.
Some reports suggest that his talks in December with powerbrokers from key poppy-producing provinces may already have led to some major cutbacks in cultivation - at least for now.
But with the drugs industry making up about 60% of Afghanistan's economy and with one in 10 Afghans involved, he could be facing a long and difficult battle.