A senior official in the US State Department has expressed growing concern at Iran's role in Afghanistan.
Assistant Secretary of State Richard Boucher said Iran might be becoming involved in an "unhealthy" way.
The remarks follow reports from Joint Chiefs Chairman Gen Peter Pace that US troops had intercepted Iranian-made arms intended for use by the Taleban.
It was not clear if the Iranian government was directly to blame, Gen Pace said.
Iran has rejected the allegations, the Associated Press news agency said.
Washington has repeatedly accused Iran of arming and training militants in Iraq and of seeking to develop nuclear weapons, charges which it denies.
But Gen Pace's remarks are the first suggestion that Tehran could be helping Afghan insurgents.
Mortars and explosives
Mr Boucher said Iran had played a positive role in Afghanistan's reconstruction and fighting the drugs trade since the fall of the Taleban in 2001.
But in the past year reports had appeared of political and military involvement, he said.
"We have been seeing a series of indicators that Iran is maybe getting more involved in an unhealthy way in Afghanistan," he said.
On Tuesday Gen Pace said coalition forces had intercepted Iranian-made mortars and plastic explosives destined for the Taleban.
But he said that unlike in Iraq it was not clear which "entity" in Iran was responsible for sending the weapons.
Iran, a Shia Islamic state, supported the Northern Alliance against the Sunni Taleban in a decade of conflict which ended with the Taleban's overthrow in December 2001.
Tehran initially cooperated with US forces fighting insurgents in Afghanistan, and as late as February this year the US military said there was no evidence Iran had sent any weapons to the Taleban.
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