The US accuses Iran of stirring up Iraq's Shia community
Tension between Iran and the United States has risen over recent days over allegations of Iranian interference in southern Iraq.
The US warned Iran against interfering with its efforts to organise a new government in Iraq, something Tehran has strenuously denied doing.
Media reports earlier this week quoted unnamed American security officials as saying that Iranian agents had crossed into Iraq, where they were trying to court Shia clerics sympathetic to Iran.
Several Iraqi opposition sources - both Shia and non-Shia - have privately expressed alarm over what they describe as "Iranian agents" in the south, working to promote an Islamic state or taking part in anti-American demonstrations.
They say these are mostly small Iraqi opposition groups which have been supported by Iran in the past.
They say there are also members of the Badr Brigade, the military wing of one of the main Shia Iraqi opposition parties, the Supreme Council for Islamic Revolution in Iraq, SCIRI.
The SCIRI is based in Tehran, but has also been involved in a US-backed process to unite the Iraqi opposition.
Some opposition sources even allege that small numbers of Iranian Revolutionary Guards have been spotted in southern cities, blending in with the crowds at demonstrations.
Iran strongly denies that any of its forces have crossed the border, although it says that members of the Badr Brigade have returned home to Iraq.
Some other Iraqi opposition groups with bases in the south say they have seen no evidence of any major Iranian infiltration.
They maintain it is unfair of the US to accuse Iran of meddling in Iraqi affairs when that is just what Washington is doing.
Iran, they point out, has sheltered large numbers of Iraqi refugees and, as a neighbour, has a legitimate interest in what kind of government emerges in post-war Iraq.
Meanwhile, a major question mark hangs over the future of the Mujaheddin-e Khalq (MKO) - Iran's main armed opposition group, which has had bases in Iraq since the 1980s.
It agreed a ceasefire with US forces earlier this week.
But the terms of that ceasefire have yet to be publicly released and there have been unconfirmed reports that MKO fighters may have been allowed to retain their weapons.
Iran has already expressed concern about such reports.
Some opposition sources say they suspect Washington could be considering using the MKO as a bargaining chip, in an attempt to persuade Iran not to encourage any form of extremism among its Shia allies in Iraq.