"They (Iran) interfere in a variety of different ways, perhaps not as violently as they do sometimes in Iraq," Richard Boucher, assistant secretary of state for south and central Asia, told reporters at a press conference.
"But what we see is Iranian interference politically, Iranian interference in terms of the money that they channel into the political process, Iranian interference in terms of playing off local officials against central government, trying to undermine the state in that way."
Boucher was speaking in Paris as part of preparations for a major international donors' conference for Afghanistan, due to take place in the French capital on June 12.
"In many ways they (Tehran) do support the work of the government, but they also work with the political opposition, they work with the local opposition," Boucher added.
"They have funnelled some weapons to the Taliban, they seem kind of working with everybody to be hedging their bets, or just looking… like they want weakness or instability in Afghanistan more than anything else."
Boucher told reporters that "several shipments" of weapons from Iran to the Taliban had been intercepted.
"I'm not sure they (Tehran) want to see the Taliban win, but I don't think they want the government to establish good control either. I think they are just trying to hedge their bets and keep everything fluid."
Boucher said that June's conference was a chance for countries to show their will to "create an Afghan government that can deliver to the people what the people want, which is safety, justice, economic opportunity, schools, health care."
France used last month's NATO summit in Bucharest to announce it would send a battalion of around 700 troops to Afghanistan, which Boucher said was a "significant contribution" to the military effort.
"The French are filling a very important gap, they are coming down in areas that are difficult," he said.
US-led forces removed the Taliban from power in Kabul in the aftermath of the September 11 attacks in 2001, but both US and NATO forces are still battling to contain an insurgency there seven years later.
