(CNN) -- Pakistan needs to do more to implement safeguards that would control a fertilizer that is being used in explosive devices in Afghanistan, according to an American lawmaker who recently visited the country.
"Basically what we're waiting for is for the Pakistani government to implement the strategy that they've said they're going to implement when we were in Pakistan in August, meeting with all the highest-level officials," said Sen. Robert Casey, D-Pennsylvania. "Unfortunately we're into October now, and I haven't seen any action in the month of September."
He said the strategy should include restricting who has access to the fertilizer; tracking it, perhaps with the use of dye; and interdicting it at the border with Afghanistan.
According to the U.S. military's Joint IED Defeat Organization, 84% of improvised explosive devices in Afghanistan are made from fertilizer produced in Pakistan, and IEDs are the No.1 killer of American forces in Afghanistan, responsible for more than 400 deaths in the past two years.
But a Pakistani official disputed Casey's criticism, saying authorities are moving as fast as they can on the problem, and that it has to be a joint effort on both sides of the border. The official, who declined to be identified, also said that out of the tons of the fertilizer produced, it is difficult to track down the small amounts placed into IEDs.
The fertilizer, calcium ammonium nitrate, is processed into small off-white pellets and is shipped in huge sacks. The same ingredient was used by Timothy McVeigh in the Oklahoma City bombing in 1995. While it has been banned in Afghanistan since 2009, it is legally produced in Pakistan and is widely used there for farming.
Calcium ammonium nitrate is "suitable for all crops, all seasons, soils and locations," according to manufacturer PakArab, whose website says they are the only firm in Pakistan that produces it.
A year ago, Afghan Border Police said they stopped a convoy in which three of the trucks were smuggling 50 tons of the fertilizer -- enough to make more than 2,100 IEDs, they said.
"It's an extremely porous border," said Seth Jones at the RAND Corp., who was in Afghanistan last month. "There's not enough time in a day, with a long line of vehicles, to stop every single vehicle and check it."
Jones said militants in Afghanistan are managing to smuggle IED ingredients up the Helmand River Valley to make roadside bombs targeting U.S. Marines operating in southern Afghanistan, as well as through areas like the Khyber Pass to target U.S. Army forces in the east.
The American pressure on Pakistan to act comes at a time when cooperation between the two countries is already under heavy strain -- most recently, after the outgoing chairman of the Joint Chiefs of Staff, Adm. Mike Mullen, said the militant Haqqani network "acts as a veritable arm of Pakistan's Inter-Services Intelligence agency." Pakistani officials have vigorously denied the accusation.
But Casey said that he would not be put off, as long as American soldiers are being injured and killed.
"Go to Walter Reed, and see those 19-year-old kids with half of their leg blown off -- or in many instances, having died this way," he said. "That's just inexcusable, when we have the resources, the technology, and the know-how, to slow this down or to stop it in some way."