Whenever a homemade bomb kills a US soldier in Afghanistan, the chances are its main ingredient came from a factory in Pakistan, a trend that has emerged as the latest source of friction in already frayed US-Pakistan relations.A US Congressional panel has frozen $700 million in aid to Pakistan until it gives assurances it is helping fight the spread of bombs in the region by regulating the distribution of calcium ammonium nitrate fertilizer from the Pakarab company. US officials have visited the complex in the dusty city of Multan over the last two years to seek the management’s help in preventing militants from smuggling fertilizer that can be used to make bombs for use over the border in Afghanistan.Company officials say they have cooperated. Bags containing ammonium nitrate, which is used in many of the homemade bombs known as improvised explosive devices (IEDs) that are so effective against US troops in Afghanistan, now have different colouring.The company is also trying to dye the fertilizer differently to help authorities track it.“It is wrong to single Pakistan and us out, especially as we are doing all that they want us to do,” Pakarab Fertilizers chief executive officer Fawad Mukhtar told Reuters. “We have told them that this is made throughout the region and Pakistan is not the only source. We have also told them that huge quantities of this fertilizer are made in Iran, Uzbekistan and across Central Asia.”But the US Congress is not in a very flexible mood and Pakarab is likely to remain in focus. US officials say the vast majority of material used to make IEDs in Afghanistan comes from Pakarab’s factories, especially the one in Multan, a city in Punjab province surrounded by cotton fields. Calls are growing in the United States to penalise Islamabad for failing to act against militant groups and, at worst, helping them, after the secret US raid on a Pakistan garrison town in which al Qaeda leader Osama bin Laden was killed in May.The freeze on US aid was agreed as part of a defence bill that is expected to be passed this week. However, securing tighter Pakistani regulation of ammonium nitrate, which is banned in Afghanistan, will be difficult.The fertilizer is churned out at the huge, grey Pakarab complex with chimneys smoking 24 hours a day. About 600,000 tonnes is produced annually.Yet company officials, fertilizer dealers and workers say the problem lies outside the heavily guarded, barbed-wired and cement walls around the complex.