The Afghan government said villagers in Nangarhar had complained of feeling sick after the mystery spraying two weeks ago, prompting Afghan officials to query U.S. and British officials.
"It is not just serious for us because of some health problems, it is not just serious for us because it harms the other crops, it is being taken very seriously because it affects the national integrity of our country," presidential spokesman Jawed Ludin told a Kabul news conference.
An investigation is continuing in Nangarhar with soil samples taken from the Shinwar and Khogyani districts.
This month, the United Nations said Afghanistan now supplies 87 percent of world opium. In 2003, the trade was worth $2.8 billion, representing more than 60 percent of gross domestic product.