Seizures in Afghanistan of ammonium nitrate fertilizer, the main explosive used in Taliban bombs, more than doubled in the first seven months of 2012 compared to the same period last year, said U.S. officials.

Despite the jump in seizures, senior U.S. officials said the number of improvised explosive devices, or IEDs, manufactured with the chemical compound has increased and is on a pace to surpass the record levels of 2011.

"We are sweeping ammonium nitrate fertilizer off the battlefield at historic rates," said a senior U.S. official who was granted anonymity to discuss sensitive intelligence. "But the IEDs are going up at historic rates too, and it is directly related. It is a supply issue."

The homemade bombs, which are most often planted along roads and footpaths, are one of the leading killers of U.S. and NATO troops in Afghanistan.

The ammonium nitrate used as the explosive component is manufactured at two plants across the border in Pakistan, and officials said the manufacturer has resisted efforts to control the flow into Afghanistan.

Figures provided to The Washington Post show that U.S. and Afghan troops have seized about 480 tons of ammonium nitrate fertilizer this year, enough explosive material to manufacture 30,000 to 50,000 IEDs.

During the same period U.S. and Afghan troops have either triggered or discovered 16,600 of the bombs, a slight increase over 2011. In June alone, U.S. and Afghan forces encountered 1,900 IEDs, a record amount in a single month for the 11-year war.

"Unless we do something about the ammonium nitrate from Pakistan we are going to continue to face these numbers and threats," said the senior U.S. official.

Sen. Bob Casey Jr., D-Pa., a member of the Senate Foreign Relations Committee, has conducted several hearings and investigations into the smuggling of ammonium nitrate from Pakistan into Afghanistan. He has pushed for a tougher stance against Pakistan for failing to curtail the trade and is troubled by the lack of progress.

The large number of IEDs uncovered this spring and summer, the traditional fighting season in Afghanistan, demonstrates that the Taliban insurgency in Afghanistan has remained resilient even as U.S. forces have increased in recent years and the territory controlled by insurgent forces has been reduced.

U.S. troop levels are on pace to shrink to about 68,000 by the end of September from a peak of about 100,000 in 2011. Yet the massive increase in explosive material flowing in to Afghanistan could make it difficult for Afghan troops to hold territory seized from the enemy in recent years once the U.S. forces have left.

Unlike their U.S. counterparts, the Afghan forces lack sophisticated technology to find and clear buried bombs. As Afghan forces have taken on a more prominent role in the fighting this year, IED attacks on Afghan troops have increased 76 percent compared with the same period in 2011, said U.S. officials.

Although U.S. casualties from the bomb blasts are down — a sign that U.S. and Afghan troops are getting better at finding the buried bombs — U.S. fatalities in Afghanistan have held steady in 2012.

Almost all of the ammonium nitrate used in the Taliban's bombs comes from two big fertilizer plants in Pakistan, both owned by the Fatima Group, based in Lahore. The production and sale of ammonium nitrate is legal in Pakistan, but it is banned in Afghanistan because of the IEDs.

Officials from the Fatima Group did not respond to a request for comment.

The vast majority of the fertilizer produced by the Fatima plants is used by small farmers in Pakistan who depend on it for their survival, said U.S. officials.

But Taliban insurgents in Afghanistan and Pakistan convert the fertilizer into explosive material that is 1.4 times more powerful than TNT.

The conversion process is fairly simple, requiring only water and a heat source. After processing, the ammonium nitrate looks like a white, powdery washing detergent and is very difficult for the largely illiterate Afghan border guard force to spot as it is being smuggled into the country.

Earlier this year, senior U.S. military officials met with Fatima Group executives to try to persuade them to add U.S.-supplied pink or yellow dyes to their fertilizer to make it easier to spot at border crossings. But Fatima Group officials rejected the American entreaties, according to U.S. officials.

U.S. officials have photographs of tens of thousands of pounds of the Fatima Group's fertilizer that has been smuggled into Afghanistan in trucks and then confiscated by U.S. and Afghan troops. The company's two multimillion dollar plants in Pakistan's Punjab province are the only facilities that are authorized by the Pakistani government to manufacture the fertilizer.

After allowing U.S. officials to tour one of the plants in 2011, Fatima Group executives have cut off contact, saying all future communications with the company must be conducted through Pakistan's Ministry of Foreign Affairs, according to U.S. officials involved in the talks.

"There is an ISI link in this," said the senior U.S. official, referring to Pakistan's powerful Inter-Services Intelligence agency. "They put the clamps on us."

Although Fatima Group is a multinational company, U.S. officials have limited tools to put pressure on the business to help control smuggling of ammonium nitrate. The U.S. Commerce Department can place foreign companies on the Entity List, which prohibits U.S. firms from doing business with the companies.

Fatima Group currently works with Bank of New York Mellon and Bank of America to facilitate trading of the company's shares in the over-the-counter market, according to Fatima Group news releases posted on the company's website.

To place a company on the Entity List, U.S. officials must be able to prove that it is knowingly selling its product to insurgent groups or terrorists. U.S. officials acknowledged that they have not been able to meet that threshold with Fatima Group.

"You have to have a witting link," said the senior U.S. official. "That is what is frustrating on this."