“I don’t believe we can significantly alter their effectiveness by cutting off their money right now,” said Representative Adam Smith, a Washington State Democrat on the House Intelligence and Armed Services Committees who traveled to Afghanistan and Pakistan last month. “I’m not saying we shouldn’t try. It’s just bigger and more complex than we can effectively stop.”

The Taliban’s ability to raise money complicates the Obama administration’s decision to deploy more United States troops to Afghanistan. It is unclear, for example, whether the deployment of 10,000 Marines over the summer to Helmand Province, the heart of the opium production, will have a sustaining impact on the insurgency’s cash flow. And American officials are debating whether cracking down on the drug trade will anger farmers dependent on it for their livelihood.

But even if the United States and its allies were able to stanch the money flow, it is not clear how much impact it would have. It does not cost much to train, equip and pay for the insurgency in impoverished Afghanistan  fighters typically earn $200 to $500 a month  and to bribe local Afghan security and government officials.

“Their operations are so inexpensive that they can be continued indefinitely even with locally generated resources such as small businesses and donations,” said Kenneth Katzman, a Middle East specialist at the Congressional Research Service and a former analyst of the region at the C.I.A.

American officials say that they have been surprised to learn in recent months that foreign donations, rather than opium, are the single largest source of cash for the Taliban.