The US says it has authorised its envoy in Baghdad to hold talks with Iranian officials about the situation in Iraq.
President Ahmadinejad had rejected previous US requests for talks
It would be the first public dialogue since the 1979 hostage crisis, after which the nations broke off ties.
Earlier, Iranian nuclear negotiator Ali Larijani said Tehran had agreed to hold talks with the US following an appeal from a prominent Iraqi Shia politician.
The White House says Ambassador Zalmay Khalilzad can talk about Iraq, but not about Iran's nuclear ambitions.
Mr Khalilzad's talks with Iranian officials will be to "express our concerns... about their involvement inside Iraq," White House spokesman Scott McLellan said.
"Those are concerns that we have expressed publicly and those are concerns that we are willing to express to them as well if they want to discuss the matter."
The question of Iran's nuclear programme, over which Tehran is facing increasing international pressure, was a "separate issue" to be dealt with at the UN, he went on.
The BBC's diplomatic correspondent, Jonathan Marcus, says that while the nations' fundamental interests differ, they may have some short-term goals in common.
The increasing violence and disorder in Iraq is making neighbouring Iran just as worried as Washington, he says.
Ali Larijani said Tehran had turned down previous requests by Washington for talks over Iraq, but had accepted the proposal to start a dialogue.
"To resolve Iraqi issues, and to help the establishment of an independent and free government in Iraq, we agree to [talks with the US]," he told reporters.
Iran's nuclear programme would be a separate issue, the US says
On Wednesday, an influential Iraqi Shia leader had urged "clear dialogue" between the US and Iran on Iraq.
"It is in the interests of the Iraqi people that such dialogue is opened," Abdul-Aziz al-Hakim said.
Zalmay Khalilzad told the BBC that Iran's role in Iraq was "mixed".
He said that although Iran said it backed Iraqi rebuilding, "it is also pursuing a complicated strategy with some extremist groups, facilitating the activities of those forces that are opposed to the system".
In the past, the US has accused Iran of backing insurgents in Iraq.
Last week the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), reported Iran to the Security Council over its controversial nuclear programme.
The Security Council starts talks this week and has the power to impose sanctions.
Iran has vowed to resist international pressure, saying that it has the right to peaceful nuclear technology.
It denies US and EU accusations that it is seeking to develop nuclear weapons.