Iranian officials, however, were adamant that they would play a mediating role on Friday.
Spokesman Dan Senor said a representative had met members of an Iranian delegation at their request and informed them that their mediation was unwelcome.
"We had a firm message for them across the board with regard to their role in Iraq, which is to be constructive, not destructive," he told reporters.
"We believe that there is no role for the Iranians to play middleman in discussions between us and al-Sadr.
"In fact we believe that the issue with Sadr and his militia should be resolved by Iraqis, not Iranians."
An Iranian foreign ministry team headed to Najaf earlier on Friday to meet Shia religious leaders, but not al-Sadr who has vowed to die as a "martyr" fighting the US-led occupation.
"We are going to Najaf to visit the holy place and we'll decide there who to meet from the religious authorities. But (al-Sadr) is out of the picture. We don't have such a mandate to meet with him," the Iranian official said.
A US-led force has deployed in strength around the Shia holy city where al-Sadr is with his armed supporters. Al-Sadr is wanted for the alleged murder of a cleric in Iraq last year.
His militia have battled occupation forces in a bloody uprising across central and southern Iraq over the past two weeks.
Al-Sadr said in an emotional sermon on Friday that a compromise with the US-led forces "will not work" and he was "yearning" for martyrdom.
"We will not allow the forces of occupation to enter Najaf and the holy sites because they are forbidden places for them," he said in his first public appearance in two weeks.
"I say that they are here to stay and will occupy us for many years and, as such, compromise will not work."