The U.S. State Department says it does not have evidence the Iranian government is involved in sending arms to the Taleban in Afghanistan.
The comments by State Department spokesman Sean McCormack appear to contradict what another top U.S. diplomat has said.
On Tuesday, U.S. Undersecretary of State Nicholas Burns said there was irrefutable evidence that Tehran is transferring arms to Taleban fighters in Afghanistan.
But spokesman McCormack told reporters Wednesday that although the United States is certain there are Iranian-made weapons in Afghanistan, U.S. officials do not know the extent of the involvement of the Iranian government.
McCormack added, however, that given what he called "the regime's past behavior" in Iraq and with the Palestinians, the issue has raised concern among U.S. officials.
Earlier this month, U.S. Defense Secretary Robert Gates said there is evidence that some Iranian-made weapons are in the hands of Taleban militants in Afghanistan. But Gates said the U.S. did not have proof that Tehran is supporting the weapons flow.
Afghan President Hamid Karzai has said there is no evidence that Tehran is arming Taleban fighters.