By Jean MacKenzie - GlobalPost

KABUL -- It is the open secret no one wants to talk about, the unwelcome truth that most prefer to hide. In Afghanistan, one of the richest sources of Taliban funding is the foreign assistance coming into the country.

Virtually every major project includes a healthy cut for the insurgents. Call it protection money, call it extortion, or, as the Taliban themselves prefer to term it, "spoils of war," the fact remains that international donors, primarily the United States, are to a large extent financing their own enemy.

"Everyone knows this is going on," said one U.S. Embassy official, speaking privately.

It is almost impossible to determine how much the insurgents are spending, making it difficult to pinpoint the sources of the funds.

Mullah Abdul Salaam Zaeef, former Taliban minister to Pakistan, was perhaps more than a bit disingenuous when he told GlobalPost that the militants were operating mostly on air.

"The Taliban does not have many expenses," he said, smiling slightly. "They are barefoot and hungry, with no roof over their heads and a stone for their pillow." As for weapons, he just shrugged. "Afghanistan is full of guns," he said. "We have enough guns for years."

The reality is quite different, of course. The militants recruit local fighters by paying for their services. They move about in their traditional 4x4s, they have to feed their troops, pay for transportation and medical treatment for the wounded, and, of course, they have to buy rockets, grenades and their beloved Kalashnikovs.

Up until quite recently, most experts thought that drug money accounted for the bulk of Taliban funding. But even here opinion was divided on actual amounts. Some reports gauged the total annual income at about $100 million, while others placed the figure as high as $300 million -- still a small fraction of the $4 billion poppy industry.

Now administration officials have launched a search for Taliban sponsors. Richard Holbrooke, U.S. special envoy for Afghanistan and Pakistan, told a press conference in Islamabad last month that drugs accounted for less of a share of Taliban coffers than was previously thought.

"In the past there was a kind of feeling that the money all came from drugs in Afghanistan," said Holbrooke, according to media reports. "That is simply not true."

The new feeling is that less than half of the Taliban's war chest comes from poppy, with a variety of sources, including private contributions from Persian Gulf states, accounting for much of the rest. Holbrooke told reporters that he would add a member of the Treasury Department to his staff to pursue the question of Taliban funding.

But perhaps U.S. officials need look no further than their own backyard.

Anecdotal evidence is mounting that the Taliban are taking a hefty portion of assistance money coming into Afghanistan from the outside.

This goes beyond mere protection money or extortion of "taxes" at the local level -- very high-level negotiations take place between the Taliban and major contractors, according to sources close to the process.

A shadowy office in Kabul houses the Taliban contracts officer, who examines proposals and negotiates with organizational hierarchies for a percentage. He will not speak to, or even meet with, a journalist, but sources who have spoken with him and who have seen documents say that the process is quite professional.

The manager of an Afghan firm with lucrative construction contracts with the U.S. government builds in a minimum of 20 percent for the Taliban in his cost estimates. The manager, who will not speak openly, has told friends privately that he makes in the neighborhood of $1 million per month. Out of this, $200,000 is siphoned off for the insurgents.

If negotiations fall through, the project will come to harm -- road workers may be attacked or killed, bridges may be blown up, engineers may be assassinated.

The degree of cooperation and coordination between the Taliban and aid workers is surprising, and would most likely make funders extremely uncomfortable.

One Afghan contractor, speaking privately, told friends of one project he was overseeing in the volatile south. The province cannot be mentioned, nor the particular project.

"I was building a bridge," he said, one evening over drinks. "The local Taliban commander called and said 'don't build a bridge there, we'll have to blow it up.' I asked him to let me finish the bridge, collect the money -- then they could blow it up whenever they wanted. We agreed, and I completed my project."

In the south, no contract can be implemented without the Taliban taking a cut, sometimes at various steps along the way.

One contractor in the southern province of Helmand was negotiating with a local supplier for a large shipment of pipes. The pipes had to be brought in from Pakistan, so the supplier tacked on about 30 percent extra for the Taliban, to ensure that the pipes reached Lashkar Gah safely.

Once the pipes were given over to the contractor, he had to negotiate with the Taliban again to get the pipes out to the project site. This was added to the transportation costs.

"We assume that our people are paying off the Taliban," said the foreign contractor in charge of the project.

In Farah province, local officials report that the Taliban are taking up to 40 percent of the money coming in for the National Solidarity Program, one of the country's most successful community reconstruction projects, which has dispensed hundreds of millions of dollars throughout the country over the past six years.

Many Afghans see little wrong in the militants getting their fair share of foreign assistance.

"This is international money," said one young Kabul resident. "They are not taking it from the people, they are taking it from their enemy."

But in areas under Taliban control, the insurgents are extorting funds from the people as well.

In war-ravaged Helmand, where much of the province has been under Taliban control for the past two years, residents grumble about the tariffs.

"It's a disaster," said a 50-year-old resident of Marja district. "We have to give them two kilos of poppy paste per jerib during the harvest; then we have to give them ushr (an Islamic tax, amounting to one-tenth of the harvest) from our wheat. Then they insisted on zakat (an Islamic tithe). Now they have come up with something else: 12,000 Pakistani rupee (approximately $150) per household. And they won't take even one rupee less."

It all adds up, of course. But all things are relative: if the Taliban are able to raise and spend say $1 billion per year -- the outside limit of what anyone has been able to predict -- that accounts for what the United States is now spending on 10 days of the war to defeat them.

VIDEO: Watch Global Post interviews with former high-ranking Taliban officials here.