But a Western diplomat based in Tehran says it's not all that clear-cut. "It seems there is a clear supply of arms to Khan," says the diplomat. "It's unhelpful, but we know the CIA, Russia, Pakistan, India, and Turkey are all dishing out cash and other things.
"Iran sees this as their sphere of influence," the diplomat adds. "There may be some fiddling on the edges, but their point of view is that if Iran didn't support the Northern Alliance for six years, there would have been no quick victory [over the Taliban]."
Part of Iran's reaction is due to the very strong presence of the US - a two-decade arch foe of the Islamic republic - in its backyard. It is a point that some in Washington fail to recognize, says Barnett Rubin, an Afghanistan-Iran specialist at New York University.
"I'm puzzled" by the US accusations, Mr. Rubin says. "The US has an interest in eastern Afghanistan. Iran has an interest in the West. But the idea that Iran is helping destabilize the regime, I find unbelievable. The Karzai government is not happy with what the US is saying about Iran."
"Khan aligned himself closer to Iran than the interim government initially - perhaps for his own security interests," says a Western relief worker who asked not to be named. "But he is a ruler in the traditional sense and is adjusting his views. Khan understands that he needs to be closer to Kabul, to comply with the rules."
Iran, adamantly opposed the Taliban and backed the rebel Northern Alliance for six years. Iranian officials point out that their support allowed the US to turn the alliance into a proxy force in the northeast of the country to help topple the Taliban last year. In the west, after Khan escaped from a Taliban prison in Kandahar in 2000, he sought exile in Iran and was allowed to muster forces among refugees there to join the battle against the Taliban on the western front.
"We want peace and stability in Afghanistan, and we are anxious about events there because they have a direct impact on our country," says Elahe Koulaiei, a member of Iran's parliamentary foreign affairs and security commission. "Our official policy is to protect the interim government, to back Karzai.
"But there are some different outlooks and views in our country - in all things," Ms. Koulaiei says, adjusting her headscarf. "With respect to the US accusation, there may be some real matters, I don't deny it."