Iraqi government spokesman Ali al-Dabbagh said the delegation had taken questions to Tehran that needed answering.
Asked if the team, led by the deputy parliament speaker, would raise the issue of arming militias, Dabbagh said:
"They will discuss all issues that have been raised ... We expect and want the intervention of Iran to be through the elected government and not through a third party."
He did not elaborate nor say who the delegation would meet.
U.S. officials say Sadr is living in the holy Iranian city of Qom, but Shi'ite officials denied reports the delegation would meet him. Sadr's spokesman denied the cleric was in Iran.
U.S. military officials had planned to put on display some of the recently captured Iranian weapons but decided to let the Iraqis make their own case to Iran first.
Maliki, a Shi'ite himself, is having to tread a fine line between Tehran and Washington - two bitter foes that are also at loggerheads over Iran's nuclear programme.
But Maliki has also sought to show his independence.
At a news conference on Wednesday, he said: "I am not Iran's man in Iraq." And he launched his offensive in the southern city of Basra without giving the U.S. military much notice.
Deputy Prime Minister Barham Salih this week described Baghdad's ties with Tehran as among the most complex it had.
"We cannot afford to have a precarious relationship that could degenerate and go back to a state of conflict that we have had in a previous era," Salih told Reuters.
"The time has come for this relationship to be put on a real sound footing, state to state."
Iran and Iraq fought an eight-year war in the 1980s in which hundreds of thousands were killed. Ties have improved since Sunni Arab strongman Saddam Hussein was ousted in the U.S.-led invasion and a Shi'ite-led government came to power in Baghdad.
U.S. forces said they killed 18 fighters in clashes beginning on Wednesday afternoon and running through the night in Sadr City, the Baghdad stronghold of Sadr. Police said two women and two children had been killed.
