MIR ALI: Pakistan's effort to cut off the flow of fertilizer to militants using it to make bombs in this key tribal sanctuary along the Afghan border has outraged local farmers, who complain the policy has cut their crop yields in half.    

The blowback in North Waziristan could prove costly as the army grapples with how to tackle enemies of the state holed up in the remote, mountainous area, a task that is likely to be more difficult if the government is unable to mobilize support from local tribesmen.

''It's true that fertilizer is being used to make bombs, but the farmers are not the ones doing it, so why does the ban apply to us?'' said Mohammad Daraz, a farmer in Miran Shah, the main town in North Waziristan.

Pakistan has struggled in recent years to avoid offending the population with heavy handed tactics as it battles domestic Taliban militants throughout the northwest.

The US has faced this same difficulty in neighboring Afghanistan, not least in its efforts to keep fertilizer, most of which comes from Pakistan, out of the hands of militants whose bombs have killed hundreds of American soldiers.

Pakistan first imposed a ban on certain types of fertilizer in North Waziristan and other parts of the semiautonomous tribal region more than three years ago, officials and farmers said.

The government instituted the policy after determining that fertilizer had been used in most of the major bombings in Pakistan, especially those involving vehicles packed with explosives, said a senior government official who worked on the ban.

The ban was meant to apply only to urea and other fertilizers that contain ammonium nitrate because they can most easily be turned into explosives, said the official, speaking on condition of anonymity because he was not authorized to talk to the media.

But security forces have instead simply tried to prevent all fertilizer from getting into North Waziristan, said farmers and fertilizer dealers.

The problem has gotten worse for the thousands of farmers in North Waziristan with each passing year as authorities have increasingly attempted to cut down on fertilizer smuggled into the area, which has become the main sanctuary for Taliban and al-Qaida militants in the country.

Most of the farmers work plots of only a few acres terraced into mountainsides or nestled in valleys next to their mud brick homes. These fields are becoming less productive because of the lack of fertilizer.

''The ban is affecting farmers, because yield is significantly reduced and crop color is faded,'' said Daraz, the farmer from Miran Shah, whose corn and wheat crops have declined more than 50 per cent.

When the government first imposed the ban, farmers were still able to buy smuggled fertilizer on the black market, although they had to pay higher prices, said Samandar Khan, a farmer in Mir Ali, another major town in North Waziristan.

The situation changed last year after security forces opened fire on a vehicle close to the North Waziristan border and wounded two people who were attempting to smuggle in fertilizer, said Rafique Ullah, a driver who has worked as a smuggler himself.

''Since then, the smugglers have almost completely stopped bringing in fertilizer,'' said Ullah. ''They are scared now because they think security forces might kill them.''

Hamidullah Khan, a farmer in Mir Ali, said there's very little fertilizer now on the black market. What is available, he said, is far too expensive for farmers because prices have risen six-fold since before the ban.

Khan said he's tried to use organic fertilizer, a mix of animal waste and rotten plants, but that his wheat crop this spring was about half the yield of most years before the ban.

''We have heard that this fertilizer is used to make bombs, but we use them for our crops,'' said Khan. ''Those who use it for bombs can purchase it even at these high prices.''

Intelligence officials denied militants were still able to obtain fertilizer in North Waziristan and said the ban has helped reduce the number of bombings in the country.

They spoke on condition of anonymity because they were not authorized to talk to the media. The spokesman for the Pakistani Taliban, Ahsanullah Ahsan, claimed the ban hasn't affected the group.

Dealers said they still managed to smuggle a few 50-kilogram (110-pound) bags into North Waziristan at a time and sell them for very high rates. They spoke on condition of anonymity for fear of being targeted by authorities.

A bag of calcium ammonium nitrate fertilizer, used by militants in both Pakistan and Afghanistan, can help produce two to four bombs, depending on whether they are targeting vehicles or foot patrols, according to the US military's Joint IED Defeat Organization.

Insurgents either grind or boil the fertilizer to separate the calcium from the nitrate, which is mixed with fuel oil, packed into a jug or box and then detonated. Urea is dissolved in water and then combined with nitric acid to make explosives needed for a bomb.