Iran and the United States have begun their first bilateral public talks for almost 30 years. Iraq's security is the only item on the agenda.
The US and Iranian envoys in Baghdad, Ryan Crocker and Hassan Kazemi Qomi are meeting in the home of PM Nouri Maliki.
The US is expected to present claims that Iran is providing technology and other support to Iraqi militia groups.
For its part, Tehran says it has uncovered several spy networks run by the US and its allies inside Iran.
On Sunday, the Iranian authorities summoned the Swiss ambassador to demand an explanation of the networks, which Iranian TV said were seeking to commit "infiltration and sabotage in western, central and south-western areas of the country".
Switzerland represents US interests in Iran.
The White House said it did not confirm or deny allegations about intelligence matters.
"We urge Iran to play a positive role in Iraq... and stop blaming everyone else for problems they are only bringing on themselves," White House spokeswoman Dana Perino said.
Monday's Iranian-US meeting is the first formal bilateral meeting since the two countries broke off relations in 1979, following the fall of the Shah.
Ryan Crocker will attend the meeting for the US side
The US has backed away from the conditions it first set for such a meeting, says the BBC's Paul Reynolds - which included support from Tehran for the government in Baghdad.
Mr Crocker will now press claims that Iran is supporting Shia and Sunni militias attacking US and British forces in Iraq.
For its part, Iran will call for a timetable for an American withdrawal from Iraq.
In a BBC interview, the Iraqi Foreign Minister, Hoshiyar Zebari, described the talks as an important first step to dialogue although he said he did not expect miracles.
But the BBC's Frances Harrison in Tehran says that although the talks are symbolically important, there is little chance of any dramatic breakthrough.
Iran's position on the talks has been dictated by the country's ultimate authority - Supreme Leader Ayatollah Ali Khamenei.
He said the aim of the meeting was to remind the American occupiers of Iraq that they had a legal responsibility to bring security to the country.
Ayatollah Khamenei says the US government is colonial, bullying, arrogant and expansionist.
But, our Tehran correspondent says, Iran is more isolated than it has been for years - now under UN as well as US sanctions because of its nuclear ambitions.
However, the nuclear issue will not be under discussion at these talks.
"Nothing but Iraq is on the agenda," Iranian Foreign Minister Manouchehr Mottaki said 10 days ago, when announcing that this meeting would take place.
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