"This is part of a considered policy, rather than the result of low-level corruption and weapons smuggling," said the NATO analysis. The document was reported by ABC News.
NATO said it had tracked weapons shipments in April and May from Iran into Afghanistan.
"We believe these intercepted munitions are part of a much bigger flow of support from Iran to the Taliban," the analysis said.
Iran and the Taliban had been enemies but they may share a common enemy.
"I think their goal is to make it very clear that Iran has the capability to make life worse for the United States on a variety of fronts even if they have to do some business with a group that has historically been their enemy," Seth Jones of the Rand Corp. told ABC.