The White House has expressed scepticism over an offer by Iran to hold talks with the US on Iraq.
President Ahmadinejad had rejected previous US requests for talks
US National Security Adviser Stephen Hadley said he was concerned it could be simply "a device" to divert pressure from Tehran's nuclear ambitions.
He said the US had suggested a dialogue last year but had received no response.
Iran's apparent enthusiasm now - days before the UN Security Council meets to discuss its nuclear programme - has raised suspicions, correspondents say.
The US ambassador to Iraq, Zalmay Khalilzad, said a potential date for what would be the countries' first public dialogue in decades was still being discussed.
Iran has confirmed the talks will focus only on Iraq. Both states say Iran's nuclear programme will not be raised.
Washington and Tehran severed relations after the Islamic Revolution in 1979.
'Drive a wedge'
Speaking to reporters, Mr Hadley said Tehran's offer could be "simply a device by the Iranians to try to divert pressure that they are feeling in New York", where the UN Security Council meets.
He also warned it was an attempt "to try to drive a wedge between the US and the other countries with which we are working on the nuclear issue".
Another senior Bush administration official called the offer "a stunt" to try to disrupt pressure on Iran from the international community.
The BBC's Jonathan Beale in Washington says Iran's announcement that it is now willing to discuss its neighbour with US officials at first took the White House by surprise.
US officials are now making clear that if there are direct talks, they will be strictly limited to the subject of Iraq, he says.
A senior US State Department official said they wanted to raise concerns over Iranian behaviour - namely about support for militants and the transfer of Iranian bomb-making equipment.
That tough line - added to Washington's suspicions - may now make it harder for any meeting to take place, our correspondent concludes.
Iranian nuclear negotiator Ali Larijani said earlier that Tehran had agreed to hold talks with the US following an appeal from a prominent Iraqi Shia politician.
Ali Larijani said Iran accepted the idea of dialogue with the US
Tehran, he said, 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.
Mr Khalilzad told the BBC that Iran's role in Iraq was "mixed".
It was supporting Iraqi reconstruction, he said, but it was also "facilitating the activities of those forces that are opposed to the system".
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 is to discuss the issue 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.