Iran has postponed talks with the US on how to end the violence in Iraq, officials announced, a day before the meeting was to be held.
Iran and the US are at loggerheads over the cause of Iraq's violence
US and Iraqi officials said Tehran had given no reason for its absence from the planned trilateral talks on Friday.
Three previous sessions have all ended in deadlock, with both sides blaming the other for the continued fighting.
The US accuses Iran of aiding radical Shia groups in Iraq, while Iran blames the violence on US presence in Iraq.
Iraqi Foreign Minister Hoshyar Zebari told Reuters news agency on Thursday: "Yesterday we were informed that the Iranians want to postpone this for some time, for some unknown reason."
There was no immediate explanation from Tehran for the postponement.
A US embassy spokeswoman in Baghdad, Mirembe Nantongo, was quoted by AP news agency as saying: "We have been saying for weeks we are ready to sit down for talks. It is increasingly clear that Iran is not."
Senior officials from the US and Iran held talks in May, July and August last year in the highest level public contacts between the two countries for 27 years.
But all three meetings ended without progress, and violence in Iraq - while at a lower level than it was in 2006 - continues.
In November the US blamed Iranian-backed militants for a bomb attack on a pet market in Baghdad that killed some 13 people.