The operation in the southern city of Kandahar, which was announced Tuesday, is by far the largest of its type. Ammonium nitrate is illegal in Afghanistan; farmers here are allowed to use other types of fertilizer, like those that are urea-based, on their crops. Most of the ammonium nitrate fertilizer in Afghanistan is believed to be imported from Pakistan.

Ammonium nitrate has long been used as both a fertilizer and an explosive. Timothy J. McVeigh and Terry L. Nichols used a 600-pound ammonium nitrate bomb, mixed with fuel oil, to attack the Alfred P. Murrah Federal Building in Oklahoma City in 1995. The attack killed 168 people.

The seizure in Kandahar came on the heels of a number of initiatives aimed at taking the fertilizer out of the hands of Taliban insurgents. Until this month, Afghan and NATO officials could seize ammonium nitrate only if it was clearly associated with insurgent activity. Now, they can seize it regardless. If the police or soldiers seize ammonium fertilizer from farmers, they are legally obliged to compensate them for it.

On Sunday, Afghan police officers and American soldiers, acting on intelligence, went first to a compound in the southern part of the city and found 1,000 100-pound bags of ammonium nitrate and 2,000 bomb-making components. They detained 15 people there. They were then led to a second compound a short distance away, where they found 4,000 100-pound bags of the fertilizer.

On Tuesday, Afghans and Americans were still carting away the ammonium nitrate; so far, officials said, they had filled 10 40-foot-long shipping containers with the bags.