The United States supports Afghanistan's desire to have productive relationships with its neighbors in order to tackle common challenges, like drug trafficking, that cross borders.
But U.S. Defense Secretary Robert Gates has accused the Iranian government of playing "a double game" in Afghanistan: offering support to the Afghan government, while at the same time providing assistance to the Taliban.
During a visit in March to Afghanistan, Admiral Mike Mullen, U.S. Chairman of the Joint Chiefs of Staff, said that Iran is having a growing, negative influence in Afghanistan. As an example, he mentioned he had recently been informed of "a significant shipment of weapons from Iran into Kandahar."
Pentagon spokesman Geoff Morrell said the Iranians are providing "low ... but consistent levels of support for Taliban fighters in Afghanistan who are clearly trying to harm our efforts and harm our forces as they go about their work:"
"That's not productive, and we clearly hope that there is a reevaluation of the kind of relationship that the Iranians want to have with Afghanistan going forward."
Mr. Morrell said it is important for Afghanistan to have positive relations with Iran, in order for the region to enjoy peace and security:
"They have long historical, cultural trade ties with their neighbors to the west, and I don't think anybody ... in this government would hope that that relationship would be a dysfunctional one."
Mr. Morrell said that in light of the controversy surrounding Iran's disputed nuclear program, Iran's activity in Afghanistan "doesn't help in their efforts to try to convince the international community that they do indeed want. . . .to contribute to the worldwide community in a positive way and that they should be trusted in what they say is their true intent." The Iranians' credibility, said Mr. Morrell, is undermined by such behavior.